The Pentagon has strongly condemned Turkey's test of the Russian S-400 air defense system, warning that defense relations between the two countries could be seriously affected as a result.

"The US Department of Defense strongly condemns Turkey's test of the S-400 air defense system on October 16," Pentagon spokesman Jonathan Hoffman said on Friday.

Hoffman stressed in a statement that his country's position regarding Turkey's possession of the S-400 system is clear and consistent, indicating that the operating system of the system does not comply with Turkey's commitment as an ally of the United States and a member of the North Atlantic Treaty Organization (NATO).

He stressed that the United States opposes Turkey's possession of the S-400 system, adding, "We object to these tests, which create serious problems for our security relations."

The spokesman referred to the suspension of Turkey's accession to the US F-35 fighter program, due to its purchase of the S-400 system.

Earlier yesterday, Turkish President Recep Tayyip Erdogan said that tests of the (S-400) missile system had already taken place, stressing that Washington's position did not matter to Ankara.

He added, "The presence of a Russian weapon (S-400) that we have is precisely what annoys some people, but we are determined on our position."

Erdogan indicated that Greece has the Russian S-300 system, and is already using it, so did America ask the Greek side about that?

Turkey and Russia concluded an agreement to supply Ankara with the S-400 air defense system, in April 2017.

And in July 2019, Washington suspended Ankara's partnership in the F-35 manufacturing project, following Turkey's receipt of Russian missile defense systems.

The Turkish army had requested the purchase of more than 100 American (F-35) combat aircraft.

Ankara was delivered two aircraft in June 2018, but they were still on US soil when the Russian S-400 system began arriving in Turkey.

Turkey has been threatened with US sanctions under a law passed by Congress in 2017 that provides for automatic punitive measures against any country that buys Russian weapons.

Congress passed a law banning the sale of F-35s to Turkey, which has produced many separate parts for this aircraft.