The United States has seen new momentum in talks with Turkey over Ankara’s efforts to return to the F-35 fighter jet program, but Washington continues to insist that Turkey must remove the Russian-made S-400 air defense system to move forward, according to US Ambassador to Turkey Tom Barrack.

In a statement on X on Tuesday Barrack said discussions between the two countries have become more constructive in recent months, thanks to what he described as an improved personal relationship between US President Donald Trump and Turkish President Recep Tayyip Erdoğan.

“As laid out in U.S. law, Turkiye must no longer operate nor possess the S-400 system to return to the F-35 program,” Barrack said, noting that the legal requirement remains unchanged despite the renewed diplomatic engagement after nearly a decade of deadlock.

The dispute dates back to Trump’s first term, when Washington removed Turkey from the multinational F-35 consortium in 2019 after Ankara took delivery of the Russian missile system despite objections from its Western allies.

The United States argues that the S-400 poses a security risk to NATO technology and could compromise sensitive data on the stealth aircraft.

The purchase triggered sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA), which Turkey remains subject to.

Barrack said the latest talks represent the “most fruitful conversations we have had on this topic in nearly a decade,” adding that officials on both sides are exploring a path that would satisfy US security demands while addressing Turkey’s desire to rejoin the program.

“Our hope is that these talks will yield a breakthrough in the coming months that meets both the security requirements of the United States and Türkiye,” he said.

This is the second time in a week that Barrack has raised the possibility of Turkey rejoining the F-35 program.

Speaking last Friday at a conference in Abu Dhabi, he said Turkey has already addressed Washington’s concerns over the operability of the Russian-made S-400 missile system because the hardware is not currently in use. But he noted that Turkey’s continued possession of the system remains a central point of tension between the two countries.

“My belief is that those issues will be resolved in the next four to six months,” he said.

When asked whether Turkey was getting closer to disposing of the Russian system, Barrack replied, “Yes.”

Since its expulsion from the F-35 program, Ankara has lobbied for reinstatement, arguing that its expulsion was unfair. The issue has resurfaced amid reports of renewed contacts between Ankara and Washington on fighter jet cooperation.

Any sale of advanced US military systems also requires congressional approval, giving lawmakers significant authority over the process at a time when both Israeli and Greek officials have voiced concerns about Turkey’s potential return to the program.

President Trump and President Erdoğan discussed the S-400 missile system during their meeting at the White House in September.

At the time Trump said he might be open to allowing Turkey to rejoin the F-35 program, saying Erdoğan was “going to do something for us,” without offering details.

Turkey’s purchase of the Russian-made S-400 system was worth an estimated $2.5 billion at the time and included two full batteries and more than 120 long-range 48N6 missiles. It is not yet known whether Moscow would be prepared to buy the systems back at a similar price, or whether Ankara would agree to sell them.

A complete removal of the S-400 infrastructure from Turkey, which would be possible through a resale to Russia, will clear one of the main obstacles to Ankara’s return to the F-35 program and reopen the possibility of acquiring the fifth-generation jets.

Before being removed from the consortium in 2019, Turkey had planned to buy 100 F-35As. Its current request is for 40 aircraft, as the country has since shifted part of its focus to developing its own stealth fighter, the fifth-generation KAAN fighter jet, which is expected to enter service in 2028. Turkey is also seeking to purchase Eurofighter Typhoons and US-made F-16s to bolster its air force.

The United States and Turkey have the two largest armies in NATO, providing critical military capabilities to the alliance.