Were US F-16 fighter jets a factor in Turkey's U-turn on Sweden's NATO bid?

President Joe Biden and Turkish President Recep Tayyip Erdogan met Tuesday amid questions about how much the potential sale of U.S. F-16 fighter jets factored into Turkey's decision to back Sweden's entry to NATO.

Biden thanked Erdogan for his "diplomacy and courage to take that on" as they held a bilateral meeting at the treaty organization's annual summit in Vilnius, Lithuania. The two leaders declined to take questions shouted from reporters before the discussion moved behind closed doors.

It was their first face to face sit-down since Turkey agreed to support Sweden joining the alliance, an unexpected move announced the day before the 74th summit kicked off.

Biden earlier Tuesday told ABC News he was "not at all" surprised by the development.

He and Erdogan spoke Sunday in an hour-long call about Sweden's membership and the issue of U.S. F-16 fighter jets for Ankara.

Turkey has long requested the jets, particularly after it was kicked out of the F-35 warplane program in 2019 for buying Russian S-400 air defense systems, but has faced opposition from U.S. lawmakers.

The U.S. was concerned Russia would learn the F-35's secrets if there was a Russian-made system in Turkey.

The current sale would include 40 new F-16s as well as kits to upgrade the jets already in Turkey's possession.

National security adviser Jake Sullivan said Tuesday the administration intends to move ahead with the sale of the jets to Turkey in consultation with Congress.

But Sullivan brushed off suggestions that Turkey lifting its opposition to Sweden's bid was linked to the transfer of F-16s.

"President Biden has been clear and unequivocal for months that he's supported the transfer of F-16s to Turkey, that this is in our national interest," Sullivan told reporters. "It's in the interests of NATO that Turkey get that capability. He has placed no caveats or conditions on that in his public and private comments over the past few months and he intends to move forward with that transfer in consultation with Congress," he said.

Sullivan didn't elaborate on a timeline for when the F-16s would be sold to Turkey, or whether it would happen before the ratification of Sweden's membership.

Secretary of State Antony Blinken has been in active conversations with congressional lawmakers regarding the potential sale, according to a senior administration official.

A bipartisan group of lawmakers in February expressed opposition to Turkey buying the jets until it signed off on Sweden's bid.

Sen. Bob Menendez, the Democratic chairman of the Senate Foreign Relations Committee, has long been against the transfer of jets to Turkey over the country's human rights record and concerns about Turkey's tensions with Greece. Menendez said Monday he's in talks with the administration about the issue.

The news that Erdogan was dropping his opposition to Sweden's entry to NATO followed a meeting between him, Stoltenberg and Sweden Prime Minister Ulf Kristersson.

In a joint statement, NATO underlined changes Sweden has made to alleviate Turkey's security concerns, including an expansion to counter-terrorism cooperation. NATO will also ramp up efforts in that arena, establishing a special coordinator for counter-terrorism.

Sweden also agreed to help Turkey's effort to join the European Union, and to increase economic cooperation between the two countries.

The NATO statement made no mention of F-16 fighter jets, and Stoltenberg on Tuesday dismissed a question of whether they had been an important factor in the final stage of the negotiations.

"There are no other agreements, the agreement is public," he responded.