The United States in September approved the $4.6-billion (4.2-million-euro) contract to deliver a total of 32 aircraft to NATO ally Poland.

The first batch is expected to be delivered within four years and will replace old Soviet models.

Poland's air force already has around 50 US F-16 fighter jets.

The agreement signed by Polish Defence Minister Mariusz Blaszczak and US ambassador to Warsaw Georgette Mosbacher includes pilot training, flight simulators and logistical support.

"This is an important day for Polish military aviation, Poland's security and also the security of our part of Europe," Polish President Andrzej Duda said at the ceremony.

The liberal opposition has slammed the purchase, claiming the fighter jets are too expensive and unsuitable for Poland's military and critics say the army is not able to make full use of the capabilities of the state-of-the-art aircraft.

The opposition has also denounced the right-wing Law and Justice (PiS) government for choosing the F-35 without a call for tenders and for agreeing to a deal without an industrial compensation programme.

Warsaw, which considers Washington its staunchest ally, has been investing in the relationship in the hopes of securing an expanded US troop presence in Poland.

Last year Poland finalised a deal to buy American mobile rocket launchers worth $414 million, while in 2018 it signed a $4.75 billion contract for a US-made Patriot anti-missile system.