"The US Department of Defense condemns in the strongest possible terms Turkey's October 16 test of the S-400 air defense system -- a test confirmed today by Turkish President Recep Erdogan," Pentagon spokesman Jonathan Hoffman said in a statement.

"We object to Turkey's testing of this system, which risks serious consequences for our security relationship," Hoffman said.

"We have been clear and unwavering in our position: an operational S-400 system is not consistent with Turkey's commitments as a US and NATO ally."

"Turkey has already been suspended from the F-35 program and the S-400 continues to be a significant barrier to progress elsewhere in the bilateral relationship," Hoffman said.

The Pentagon reaction came several hours after Erdogan confirmed that tests of the S-400, which was delivered to Turkey by Russia last year, had been carried out by the Turkish armed forces.

"It is true about the tests, they have been done and will continue," Erdogan told reporters in Istanbul. "We're not going to ask America for permission."

Erdogan stressed that the US stance on the S-400 issue "did not bind" Turkey, adding: "If we're not going to test what we have in our hands, what else would we do?"

There had been hope in Washington that Ankara would keep the missile system "in the box" but Turkey has always insisted the S-400s would be deployed after their delivery last year.

The S-400 test came despite repeated warnings of sanctions from the US State Department if the system was activated.

Ankara has accused Washington of failing to sell it competing American Patriot missile defense batteries and points to its security needs for purchasing the Russian system.

Turkey faces potential sanctions under a 2017 law known as CAATSA, which mandates sanctions for any "significant" purchases of weapons from Russia.

Turkey had already been removed from the F-35 fighter jet program over the S-400 purchase.

- Military exercises cancelled -

The S-400 tests come at a particularly tense time in Turkey's relationships with NATO allies the US, France and Germany, after Ankara resumed gas exploration this month in waters disputed by another alliance member, Greece.

NATO Secretary-General Jens Stoltenberg said Friday that Turkey and Greece have agreed to cancel rival military exercises that were to have been held next week on their respective national days.

"This is a very welcome step," Stoltenberg said after a videoconference of NATO defence ministers, including Greece's Nikos Panagiotopoulos and Turkey's Hulusi Akar.

"These are steps in the right direction, and it helps to reduce the risks for instance and accidents," Stoltenberg said.

The neighbours, while NATO members, are at loggerheads over energy drilling and maritime rights in the eastern Mediterranean and the alliance has set up a hotline to head off accidental clashes.

Greece had been expected to conduct exercises on Wednesday, October 28, its Oxi Day holiday, and Turkey on Thursday, celebrated there as Republic Day.

Turkey has deployed the Oruc Reis, a gas exploration vessel under military escort, into Greek waters off the island of Kastellorizo, and Greek vessels are nearby.

French Defence Minister Florence Parly also hailed the decisions to cancel the military exercises, stressing the need to "respect international law and restore stability in the region."