The Kremlin said on Monday that Russia and the United States were working on unspecified ideas on a possible peace settlement in Ukraine after U.S. President Donald Trump said he was "pissed off" at Russian President Vladimir Putin.

Trump told NBC News he was very angry after Putin last week criticised the credibility of Ukrainian President Volodymyr Zelenskiy's leadership, the television network reported, citing a telephone interview early on Sunday.

Trump said he would impose secondary tariffs of 25% to 50% on buyers of Russian oil if he felt Moscow was blocking his efforts to end the war in Ukraine.

Asked about Trump's comments, Kremlin spokesman Dmitry Peskov said Putin remained open to contacts with Trump and that a call between them could be set up at short notice if necessary though no call was scheduled for this week.

Peskov said Moscow and Washington were continuing to work on building up bilateral ties.