The Biden administration believes a potential sale of F-16 fighter jets to Turkey would be in line with US national security interests and serve Nato's long-term unity, the US State Department has said in a letter to Congress.

Though the letter did not go so far as to explicitly support the deal, Ankara will take it as progress in its long drawn-out attempt to upgrade its airforce to the levels being attained by regional rivals such as Greece. Nato member Turkey's move to acquire more than 100 of the world's most advanced fighter aircraft, the F-35 stealth jet, remains frustrated—its order has been blocked by Washington in response to Ankara's purchase from the Kremlin of S-400 advanced missile defence systems, an acquisition the US and Nato advised against, fearing the Russian technology could access sensitive information on the performance of Nato military hardware such as the F-35.

Turkey last October requested that the US grant it permission to buy 40 Lockheed Martin-made F-16 fighters and nearly 80 modernisation kits for its existing warplanes. Washington has previously not stated any opinion on the sale, simply saying it needed to pass through the standard arms sales process.

The State Department letter, first reported by Reuters, is dated March 17 and signed by the agency's top legislative official Naz Durakoglu. She acknowledged strained relations between Ankara and Washington but noted that Turkey's support for and defence ties with Ukraine were "an important deterrent to malign influence in the region." Ukraine has acquired at least two dozen Bayraktar combat drones from Turkey.

Washington's punitive actions after Ankara's purchase of the S-400 systems represent "a significant price paid", the letter added.

"The Administration believes that there are nonetheless compelling long-term NATO alliance unity and capability interests, as well as U.S. national security, economic and commercial interests that are supported by appropriate U.S. defense trade ties with Turkey," it also said, adding: "The proposed sale will require a Congressional notification if the Department of State were to approve it."

The State Department letter was in response to a Feb. 4 letter led by Democratic congressman Frank Pallone and signed by more than 50 lawmakers from both the Democratic and Republican parties. The letter lobbied the Biden administration to reject Ankara's purchase plan for F-16s, pointing to what was deemed Turkish President Recep Tayyip Erdogan's lack of commitment to Nato and his "vast human rights abuses."