A bipartisan group of 29 senators called on US President Joe Biden to delay the $20 billion sale of F-16 fighters to Turkey until Ankara agrees on the accession of Sweden and Finland to NATO.

Last May, Finland and Sweden handed in their applications to join the US-led military alliance NATO and were later formally invited by the alliance to join.

However, their bids for accession require the unanimous approval of all 30 NATO member states to be considered. Hungary and Turkey are the only countries to have not approved the applications.

Last October, Turkey made a request to purchase 40 Lockheed Martin F-16 fighter jets and 80 modernization kits for their already existing warplanes.

The letter comes after CNN-Turk reported last month that there is a possibility that the United States may finally approve Turkey's request to buy F-16 fighter jets, adding strength to NATO allies in the Black Sea. 

Read more: Turkey may turn to Russia if US blocks F-16 sales: Erdogan

Turkey demanded that the Nordic country take harder stances against the Kurdistan Workers' Party (PKK), considered a terrorist group by Ankara and the EU, in addition to another group that Turkey blames for the 2016 alleged coup attempt.

The Democratic and Republican senators sent a letter to Biden saying that both Sweden and Finland were taking "full and good faith efforts" to meet the requirements to join the military alliance that Ankara asked for, despite Turkey asking Stockholm for more concessions.

Read more: US arming Kurdish groups in Syria as Turkey threatens ground operation

"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 letter read.

This would be the first time the US Congress directly links the sales of the jets to Ankara with the two countries' NATO bid.

Earlier this month, Turkish Foreign Minister Mevlut Cavusoglu said Ankara is awaiting US approval for a request for F-16 fighter jets.

Cavusoglu said during a meeting with US Secretary of State Antony Blinken that the F-16 agreement should proceed separately to Finland and Sweden's NATO accession.

On January 14, Chairman of the US Senate Foreign Relations Committee, US Senator Bob Menendez, vowed to block a proposed sale of F-16 fighter jets to Turkey unless Ankara "adjusts its behavior" to suit Washington’s tastes.

The US legislature's leading foreign affairs official pledged to block the deal, accusing Turkish President Recep Tayyip Erdogan of attempting to "undermine international law, disregard human rights and democratic norms, and engage in alarming and destabilizing behavior in Turkey and against neighboring NATO allies."

Following the burning of the Quran in Sweden last month and the incident where an effigy of Turkish President Recep Tayyip Erdogan was hung by its feet at a protest staged in the Swedish capital by PKK supporters, tensions have been at their highest between Ankara and Stockholm, while Turkey said it will consider backing Finland's accession to NATO if it submits an application without Sweden.

Read more: Sweden took no steps to fulfill obligations on NATO accession: Ankara