WASHINGTON, Feb 2 (Reuters) - The U.S. Congress cannot support the $20 billion sale of F-16 fighter jets to Turkey until Ankara ratifies the NATO memberships of Sweden and Finland, a bipartisan group of senators said on Thursday.

Sweden and Finland applied last year to join the trans-Atlantic defense pact after Russia invaded Ukraine, but faced unexpected objections from Turkey and have since sought to win its support.

Ankara wants Helsinki and Stockholm in particular to take a tougher line against the Kurdistan Workers' Party (PKK), which is considered a terror group by Turkey and the European Union, and another group it blames for a 2016 coup attempt.

The three nations reached an agreement on a way forward in Madrid last June, but Ankara suspended talks last month following protests in Stockholm in which a far-right Danish politician burned a copy of the Muslim holy book, the Koran.

In a letter to President Joe Biden, 29 Democratic and Republican senators said the two Nordic countries were making "full and good faith efforts" to meet the conditions for NATO membership that Turkey asked, even though Ankara says Sweden needs to do more.

"Once the NATO accession protocols are ratified by Türkiye, Congress can consider the sale of F-16 fighter jets. A failure to do so, however, would call into question this pending sale," the senators wrote.

It was the first time Congress explicitly and directly linked the F-16 sale to Turkey with the NATO accession bids of the two Nordic countries.

The Biden administration has repeatedly said it supports the sale and refused to link the two issues, although it acknowledged that the ratification of Sweden and Finland's NATO accession would facilitate the sale process in Congress.

Turkey has said it could approve Finland's NATO membership application ahead of Sweden's, but the Finnish president and foreign minister have both rejected this idea, arguing that the security of the two Nordic countries is mutually dependent.

Of NATO's 30 members, only Turkey and Hungary have yet to ratify the Nordic countries' memberships.

Turkey requested in October 2021 to buy 40 Lockheed Martin Corp (LMT.N) F-16 fighters and nearly 80 modernization kits for its existing warplanes.

In a visit to Washington last month, Turkish Foreign Minister Mevlut Cavusoglu said the NATO issue should not be a precondition for the sale and urged the Biden administration to persuade Congress to drop its objection.

While Congress can block foreign arms sales, it has not previously mustered the two-thirds majorities in both chambers required to overcome a presidential veto.