Greece is expected to send an official request in the coming days to the United States for the purchase of 20 Lockheed Martin made F-35 fighter jets, defence sources said on Thursday.

NATO member Greece spends more than 2% of its gross domestic product on defence spending. It has beefed up its military purchases in recent years as tensions with its neighbor, historic rival and NATO ally, Turkey have resurfaced.

"The relevant document has been signed," one of the sources said. Deliveries of the jets will begin in 2027-2028.

According to state TV ERT the Letter of Request has been signed and is expected to be submitted to the U.S. Embassy in Athens this week. This is the first step in a multi-stage process.

Greece and Turkey have been at odds for decades over a series of issues ranging from overflights in the Aegean and the extent of their continental shelves and their maritime boundaries to energy resources and ethnically-split Cyprus.

Greek Prime Minister Kyriakos Mitsotakis referred to the purchase of F-35 jets during a visit in May to the United States.

Athens ordered 24 Dassault-made Rafale jets last year for 2.5 billion euros and three navy frigates with an option for a fourth one from France for about 3 billion euros.

According to state ERT TV, the defence ministry also wants to proceed with the upgrade of F-16 fighter jets.

On Wednesday, the United States supported the potential sale of U.S. F-16 fighter jets to Turkey, a day after Ankara lifted a veto of NATO membership for Finland and Sweden, saying that strong Turkish defense capabilities would reinforce NATO's defences. (Reporting by Renee Maltezou and Lefteris Papadimas, Editing by Angus MacSwan)