Turkey initially purchased the S-400 in 2017, plunging its relationship with the U.S. into crisis. As a result of the deal, the Trump administration expelled Turkey from the F-35 program and later sanctioned the country’s defense industry organization and its leaders. The United States fears the powerful S-400 radar system would allow Russia to spy on the advanced F-35 fighter jet.

But the reaction in the U.S. on Tuesday was comparatively muted. At a press briefing, State Department spokesman Ned Price urged Turkey not to further engage with Russia’s defense sector.

“The point that we have consistently made across the board is that Russia’s brutal and unjustified war against Ukraine makes it vital, now more than ever in some ways, that all countries avoid transactions with Russia’s defense sector,” he said.

“We’ll have to wait and see what happens, but we are not aware of any new developments on this matter,” Price added.

Price declined to address whether a new S-400 would prompt the Biden administration to reconsider plans to sell F-16 fighter jets to Turkey.

The Russian news agency TASS reported Tuesday Russia and Turkey have signed a contract for a second “regiment” of the S-400 to Turkey, quoting the head of the Federal Service for Military-Technical Cooperation, Dmitry Shugayev.

Turkish defense sources quickly rebutted Russian claims it had signed an additional contract, noting the second S-400 was part of the initial agreement.

Shugayev, the Russian official, reportedly told TASS the new agreement with Turkey will allow for some components of the system to be built in Turkey.

Turkish sources told Defense News there may be an agreement with Russia to domestically manufacture some S-400 components for the second regiment.

The Turkish Embassy in Washington didn’t return a request for comment.

Turkey’s complex balancing act between the West and Russia has only gotten trickier since Russia’s invasion of Ukraine. The strongest sign of a thaw between Turkey and the U.S. came in late June as Turkish leader Reccep Tayyip Erdogan dropped objections to Finland and Sweden joining NATO.

U.S. President Joe Biden then voiced support for the F-16 sale at the NATO summit in Madrid in June.

A delegation from Ankara arrived in the U.S. on Monday for the fourth round of technical talks with Washington on a potential F-16 deal, Turkey’s Anadolu Agency reported this week.

Turkey has asked to make a $6 billion purchase of 40 Lockheed Martin Block 70 F-16 fighter jets. It is also seeking a separate $400 million sale to upgrade their current F-16 jets with new missiles, radar and electronics.

Senate Foreign Relations Committee Chairman Bob Menendez, D-N.J., has threatened to block the F-16 sale over Turkey’s continued possession of the S-400s, its violations of Greek airspace and Cypriot waters in the eastern Mediterranean, its human rights record and ongoing attacks on U.S.-backed fighters in northeast Syria.

Menendez issued a statement on Thursday blasting reports that Turkey intends to proceed with the second S-400 batch and called on Ankara to “unequivocally demonstrate its commitment to NATO and to regional peace and security by fully rejecting any military cooperation with a war criminal like Vladimir Putin.”

“Between continued antagonistic violations of Greece’s airspace and the delaying of the accession process for Sweden and Finland’s NATO membership, I sincerely hope Turkey will change course and live up to its responsibilities to the defense alliance by being the constructive partner in the region we all hope it can be,” said Menendez. “The United States must be clear: Any expansion of Turkey’s ties to the Russian defense sector would be a grave mistake that would further endanger the security of our NATO allies and partners throughout Europe.”

Despite increased tensions with its NATO allies in recent years, Turkey has generated some goodwill in Washington over its support for Ukraine against Russia. Turkish firm Bayraktar has been selling Ukraine its TB2 laser-guided, bomb-dropping drone for use against Russian troops, and, in June, Ukraine said Bayraktar would open a drone factory within its borders to help repel the invasion.

In recent weeks, Erdogan also brokered an international deal with Russia to allow the export of grain from Ukraine, a breadbasket for the world, to ease a global food crisis. Agricultural goods had been stuck in Black Sea ports due to the invasion.

Eric Edelman, a former U.S. ambassador to Turkey and senior Pentagon official, said he believes the S-400 deal between “competimates” Erdogan and Russian President Vladimir Putin will confound efforts to mend ties between Turkey and the U.S.

“To say the least, this will complicate the Biden administration’s effort to sell F-16s to Turkey and enrage Turkey’s growing legion of detractors on Capitol Hill,” Edelman said.

Jim Townsend, the Pentagon’s top Europe and NATO policy official during the Obama administration, said he doubts the S-400 deal came about without Turkey giving advanced warning to the U.S. He hypothesized that it was a component of the international grain deal.

“This might be residue from that deal, and if that is the case, then we probably heard about it, not only because we were probably in the background of these discussions, but also so that the Turks don’t have to pay a price from the U.S. for doing something to make the grain deal work,” Townsend said.

The hushed U.S. reaction to the Russian report is a clue, he added.

“If we’re not thundering from the pulpit, we probably knew about it,” Townsend said.

Turkey committed to buy more than 100 F-35s, but was removed from the program after it decided to purchase the Russian-made S-400 long-range air and anti-missile defense system.

Erdogan has been pressing the Biden administration either to deliver the two F-35s built for the Turkish military or return Turkey’s $1.4 billion payment for the aircraft.

Turkey’s current fleet of more than 200 F-16s will be phased out by 2035. Nearly 100 aircraft in the fleet await upgrades.

Turkey also is struggling to build the TF-X, its first indigenous fighter jet, and is aiming to fly the stealth aircraft in 2025 or 2026.

ANKARA, Turkey — If U.S. officials were to expel Turkey from the multinational group that builds the F-35 Lightning II, Turkish defense officials said they likely would pursue Russian fighter jet technology.

“We cannot afford to leave the F-35 not substituted,” a senior military officer told Defense news. He declined to comment on the replacement options, as this would require “technological, economical and political deliberations.”

But a defense procurement official said “geostrategic assessment” would make Russian options emerge as the natural first replacement. “Russian fighter technology would the first best choice if our American allies behaved in an un-allied way and questioned Turkey’s membership in the Joint Strike Fighter program,” he said.

Washington has threatened to expel Ankara from the multinational program if Turkey deploys the Russian-made S-400 surface-to-air missile system on its soil.

If Turkey accepts the S-400, “no F-35s will ever reach Turkish soil. And Turkish participation in the F-35 program, including manufacturing parts, repairing and servicing the fighters, will be terminated, taking Turkish companies out of the manufacturing and supply chain for the program,” wrote a group of bipartisan lawmakers from the Senate Armed Services Committee and Senate Foreign Relations Committee.

Earlier this month, the Pentagon announced a freeze on deliveries and activities with Turkey in relation to the F-35 program over Ankara’s decision to buy the S-400. Turkey insists the first S-400 shipments would arrive in July and the first S-400 system would become operational in September.

A Turkish presidential source said that potential Turkish-Russian cooperation on fighter technology was “preliminarily discussed” between their respective defense officials during Turkish President Recep Tayyip Erdogan’s visit to Moscow on April 8.

Turkish Foreign Minister Mevlut Çavusoglu told broadcaster NTV on Apri 10: “There are F-35[s], but there are also aircraft manufactured in Russia. If we are not able to purchase [the] F-35, Turkey will buy similar aircraft from other countries. And this will continue until we start producing our own fifth-generation fighter jets.”

Such a move would make Turkey the only NATO member to simultaneously use the S-400 and Russian fighter.

“This is not a useful dispute for the alliance,” asserted an Ankara-based European Union military attache. “What we observe today could push Turkey further into Russia’s military orbit … and we don’t want that.”

Earlier this month, Russian Helicopters CEO Andrey Boginsky visited Turkey to discuss the possibility of co-production efforts.

However, Russia’s isn’t the only alternative for the F-35. In 2015, Turkey’s procurement authorities released a request for information for the TF-X, the country’s indigenous fighter jet program. Sweden’s Saab was one of the bidders to supply know-how for the initial design phase of the program, but Ankara selected Britain’s BAE Systems for that contract.

“Saab’s commitment to technology transfer was very generous, but its price was expensive at the time,” a Turkish official recalled. “Saab could now revise its bid and incorporate it into the new [no F-35] situation,” he said.

Another option for Turkey is Airbus, a partner in the Eurofighter program based in the Netherlands and France, the official added.

Separately, the Turkish government congratulated Trump for his presidential victory while repeating its call for the extradition of US-based Islamic preacher Fethullah Gulen, accused of orchestrating the failed coup attempt of July 15.

Prime Minister Binali Yildirim said: “We congratulate Mr. Trump. I am openly calling on the new president from here about the urgent extradition of Fethullah Gulen, the mastermind, executor and perpetrator of the heinous July 15 coup attempt, who lives on US soil.” The prime minister said Gulen’s extradition could mark a new beginning in U.S.-Turkish relations.

“I am sure that the strategic partnership of Turkey and the US, which is based on a long history, has created an opportunity for the new president, paying attention to Turkey’s sensitivity in the fight against terrorism, carrying forward traditional friendly relations, [and] prioritizing peace and security in the region,” Yildirim added.

Turkey has been vehemently demanding Gulen’s extradition from the US since the failed putsch of July 15. Gulen, once a staunch political ally of Erdogan, has been living in self-exile in Pennsylvania since 1999.

Procurement officials in Ankara said the future of defense cooperation would largely depend on whether Trump would favour a policy for more technology transfers to Turkey.

“We do not think Trump’s election will be an immediate game-changer in procurement ties,” one official said. “Over time, however, we would expect him to pursue a policy to share more with NATO ally Turkey.”