Turkey accused the United States on Tuesday of not acting as an ally, in its official response to Washington's ultimatum to Ankara to abandon a deal to buy a Russian missile defense system.

The Turkish defense ministry said in a statement that 'the wording and approach' of a letter sent this month by Acting US Defense Secretary Patrick Shanahan 'was not in the spirit of an alliance (and) caused unease'.

The ministry said Turkey's previously 'known' opinions on acquiring the Russian S-400 missile defense system had been shared in a detailed manner, without offering further information.