By Steve Holland and Justyna Pawlak

VILNIUS (Reuters) -Washington will move ahead with the transfer of F-16 fighter jets to Turkey in consultation with Congress, U.S. National Security Adviser Jake Sullivan said on Tuesday, a day after Ankara gave the green light for Sweden to join NATO.

Turkey, which had been the main stumbling bloc on Sweden's path towards the alliance, had requested in October 2021 to buy $20 billion of Lockheed Martin Corp F-16 fighters and nearly 80 modernisation kits for its existing warplanes.

Speaking ahead of a summit of NATO leaders that started on Tuesday in the Lithuanian capital Vilnius, Sullivan said U.S. President Joe Biden "had been clear that he supports the transfer".

"He has placed no caveats on this ... He intends to move forward with that transfer," Sullivan told reporters, without giving any details on the timing.

U.S. Senate Foreign Relations Committee Chairman Bob Menendez, a Democrat who had blocked the transfer, said on Monday he is in talks with the Biden administration about his hold and that he could make a decision "in the next week", suggesting he could lift it.

Both Turkish officials and the Biden administration have rejected any suggestion that Ankara's approval of Sweden's NATO accession was being linked to the F-16 sale in the months of talks to address Turkish opposition.

However, some diplomats and analysts believe that Turkish President Tayyip Erdogan had been using Swedish membership to pressure Washington on the warplanes, and that Biden made a deal.

"There seems to have been a big push by the Biden administration to allow Turkey to modernise its air force and acquire new F-16s," said Camille Grand, a defence specialist at the European Council on Foreign Relations think tank.

"This push together with the Swedish efforts on the PKK front might have played an important role in convincing Erdogan to move forward on Sweden."

One former French ambassador to Washington, Gerard Araud, wrote on Twitter in response to Sullivan's announcement that "'The Swedish blackmail' paid".

To explain its opposition to Sweden's NATO accession, Ankara had accused it of not doing enough against people Turkey sees as terrorists, mainly members of the outlawed Kurdistan Workers Party (PKK) that is considered a terrorist organisation by Turkey, the EU and the United States.

A statement issued by Turkey and Sweden on Monday said Sweden had reiterated that it would not provide support to the Kurdish groups and would actively support efforts to reinvigorate Turkey's EU accession process.

Russian officials said Sweden's expected accession to NATO would have "negative implications" for Russia's security and that Moscow would have to respond.

TIMING UNCERTAIN

The timing of both the F-16 transfer and Sweden's NATO entry remains unclear.

Turkey's parliament is not scheduled to convene until after the summer, and Hungary also still needs to ratify the accession treaty, although Foreign Minister Peter Szijjarto said in a statement on Tuesday that it was "only a technical issue".

All NATO states need to ratify a new member.

NATO Secretary-General Jens Stoltenberg said Erdogan had agreed to push ratification in parliament "as soon as possible", but he could not give a specific time frame.

It took two weeks for Turkey's parliament to ratify Finland's membership. Finland had applied alongside Sweden after Russia's invasion of Ukraine in February 2022 overturned the two Nordic nations' security considerations.

(Reporting by Steve Holland, Justyna Pawlak; editing by John Irish, Nick Macfie, Heather Timmons and Devika Syamnath)