Washington has warned for months that Turkey's adoption of the Russian S-400 missile system would endanger Western defense and jeopardize Ankara's planned purchase of 100 of the US's F-35 stealth fighter jets.

Asked on Thursday if Turkey may ultimately change its mind on the S-400, deputy US defense secretary Heather Wilson replied "it's possible."

"The diplomats are continuing the work on that," she said, reiterating that the S-400 is "incompatible with having the F-35."

The US in April placed a freeze on a joint F-35 manufacturing program with Turkey, and US law furthermore provides for sanctions on any country concluding arms deals with Russian companies.

Two of the planes were delivered to Turkey in June 2018 but remain at a US Air Force base near Phoenix, Arizona, officially so Turkish pilots can train on them.

"We're continuing to train the Turkish pilots at Luke Air Force Base but we don't think that we can deliver those aircraft into a country that has the S-400," Wilson said.

Ankara says it won't reconsider purchasing the S-400 air defense system and that delivery of the first of the equipment may come as soon as June or July.

But Foreign Minister Mevlut Cavusoglu said last month that Turkey is aware of the US's concerns.

Turkish media has reported that the government is mulling not using the Russian batteries or selling them to a third party in order to resolve the dispute.