The Trump administration still plans to impose sanctions on Turkey and remove it from a critical fighter jet program if the NATO ally acquires Russian air defenses, US officials told Reuters, despite the Turkish president's assurances to the contrary.

After meeting US President Donald Trump over the weekend in Japan, Turkish President Tayyip Erdogan said Ankara would be spared damaging US sanctions once Russia's S-400 air defense system starts arriving in Turkey in coming days.

Trump appeared sympathetic to Erdogan at the talks and reluctant to publicly commit to sanctions - despite being repeatedly asked by reporters.

But US government officials told Reuters that, at least so far, the administration intends to impose sanctions on Turkey and pull it from the F-35 fighter jet program if it takes delivery of the Russian S-400 system, as expected.

"The United States has consistently and clearly stated that Turkey will face very real and negative consequences if it proceeds with its S-400 acquisition, including suspension of procurement and industrial participation in the F-35 program and exposure to sanctions under the Countering America's Adversaries through Sanctions Act(CAATSA)," a State Department spokeswoman said.

At the Pentagon, spokesman Air Force Lieutenant Colonel Mike Andrews said: "Nothing has changed."

"Turkey's purchase of the Russian S-400 air and missile defense system is incompatible with the F-35 program. Turkey will not be permitted to have both systems," Andrews said.

If the US removes Turkey from the F-35 program, and imposes sanctions on the NATO ally, it would be one of the most significant ruptures in recent history in the relationship between the two nations.

Trump, who has shown a rapport with Erdogan, could still try to change course by seeking to issue a waiver and postpone sanctions. Such a move would please Ankara but upset some of Trump's allies in Congress.