US Secretary of State Antony Blinken said on Wednesday that Turkey and all US allies should refrain from buying more Russian weapons, at the risk of more sanctions being imposed.
Frayed relations between the US and Nato ally Turkey were further tested at the weekend after President Joe Biden recognised the Armenian massacres in 1915 as genocide, infuriating Ankara.
Mr Blinken said that given Mr Biden's long-standing views on the Armenian issue, his decision should not have been a surprise.
He also repeated that Turkey was a critical Nato ally and expressed his hope that the two sides could resolve their issues.
But he warned Ankara and others not to buy more military equipment from Russia. Turkey has said it is in talks with Russia for a second batch of S-400 air-defence missiles.
"Any significant transactions with Russian defence entities, again, could be subject to the law, to CAATSA, and that's separate from and in addition to the sanctions that have already been imposed," Mr Blinken said.
He was referring to the Countering America’s Adversaries through Sanctions Act, which is designed to dissuade countries from buying military equipment from Russia.
US-Turkish relations have been strained over issues such as Turkey's purchase of S-400s, making it a target for US sanctions, and policy differences on Syria, human rights and a US court case focused on Turkey's majority state-owned Halkbank.
Washington in December imposed sanctions on Turkey for buying the Russian air defence system.
Ankara has expressed anger that the US armed Kurdish YPG fighters in Syria and has not extradited a cleric living in the US, who Turkey has accused of orchestrating a 2016 coup attempt.
Mr Biden, in his first call as president with Turkish leader Recep Tayyip Erdogan on Friday, told him about his decision on the Armenian genocide.
Mr Blinken said the two leaders had a "good conversation" and that Mr Biden was looking forward to meeting Mr Erdogan in June at the Nato summit.
Turkey's presidential spokesman said on Sunday that Mr Biden's genocide declaration was "simply outrageous" and Turkey would respond over the coming months.
