The United States on Friday warned Turkey of further risks to bilateral ties if it buys more weapons from Russia, after US countermeasures over its purchase of the S-400 air defense system.

The statement came after Turkish President Recep Tayyip Erdogan met with his Russian counterpart Vladimir Putin earlier this week and discussed greater military cooperation, including on warplanes and potentially submarines.

Turkey, a NATO member, defied US warnings in 2017 to buy the S-400 air defense system, a deal that the Western alliance fears will allow Russia to hone its skills at targeting US and Western European aircraft.

"We've urged Turkey at every level and opportunity not to retain the S-400 system and refrain from purchasing any additional Russian military equipment," Deputy Secretary of State Wendy Sherman told reporters when asked about Erdogan's trip.

"We continue to make that clear to Turkey and what the consequences will be if they move in that direction," said Sherman, who was on a visit to Switzerland.

Sherman reiterated that the S-400 was "not compatible or operable with NATO systems."

In response, the United States expelled Turkey from its state-of-the-art F-35 jet program in which multiple Turkish companies were involved and through which Ankara expected as many as 100 planes and had Turkish suppliers.

But Sherman stressed that the United States valued the relationship with Turkey, pointing to its welcome to millions of Syrian refugees and offers to help restore Kabul's international airport.

"Turkey is a challenge sometimes. I'm sure they find the United States a challenge at times. But they are a valued NATO ally," Sherman said.

Erdogan said Thursday that his relationship has "not gotten off to a good start" with US President Joe Biden, who while out of power described the Turkish leader as an autocrat and voiced support for the opposition.