US President Donald Trump has said he expects Russian President Vladimir Putin "to fulfil his part" of a deal to end the Ukraine war after he voiced doubts over Moscow's commitment.

"I want to make sure that he follows through," Trump told reporters in the White House on Monday. "I think he's going to live up to what he told me, and I think he's going to fulfil his part of the deal now."

Trump on Sunday criticised both Putin and Ukraine's Volodymyr Zelenskyy as he pushes to end more than three years of conflict in Ukraine.

Trump told NBC he was "very angry" at Putin for questioning Zelenskyy's credibility as someone to negotiate with and for calling for new leadership in Ukraine.

The US leader later turned his ire on Zelenskyy, warning him Ukraine would have "big problems" if it got cold feet over a deal to sign over mineral rights to the United States.

The Kremlin on Monday said that Putin was still open to speaking to Trump.

There was no phone call scheduled between the two leaders, but that one could be "organized promptly," Kremlin spokesman Dmitry Peskov told reporters.

"We are... working on implementing some ideas, linked to resolving the Ukrainian (conflict)," Peskov said. "This process is time-consuming."

'Normalisation brings optimism'

Chinese Foreign Minister Wang Yi, during a visit to Moscow, hailed the steps towards normalising ties between the US and Russia, saying it inspires "optimism."

"Russia and the United States have taken the first step toward normalising relations, which is good for stabilising the balance of power between major powers and inspires optimism in a disappointing international situation," RIA cited Wang as saying.

China calls for the peace talks to continue, Wang told RIA in a wide-ranging interview.

"The step towards peace, although not that big, is constructive — it's worth building on it," Wang said.

"With peace, it is no pain, no gain. You need to work hard to achieve it."