US President Donald Trump has said he would meet with Vladimir Putin even if the Russian leader didn't sit down beforehand with Ukrainian President Volodymyr Zelensky.

The remark contradicted earlier reports in US media which, citing unnamed White House officials, said a Putin-Zelensky meeting was a prerequisite for Trump's own summit with his Russian counterpart.

Asked by reporters in the Oval Office if Putin was required to meet Zelensky first, Trump said simply: "No, he doesn't, no."

"They would like to meet with me, and I'll do whatever I can to stop the killing," he added.

A day earlier, Trump said that a face-to-face meeting with Putin could occur "very soon," after his envoy Steve Witkoff met the Russian leader in Moscow.

Trump has been trying since taking office in January to end Russia's military assault on Ukraine, after boasting on the campaign trail that he would end the conflict within 24 hours.

He has recently increased pressure on Russia, issuing a Friday deadline for Moscow to reach a ceasefire deal or face increased penalties.

Asked on Thursday if the deadline still stood after the talks with Witkoff, Trump did not answer directly.

"It's going to be up to (Putin)," Trump said. "We're going to see what he has to say."

"Very disappointed," he added.

RTHK's Washington correspondent, Simon Marks, said that many uncertainties lie ahead of the meeting.

"There are real questions again about the format of these talks and about whether the United States is placing any kind of preconditions on them at all," he told RTHK's Hong Kong Today programme.

"But if Donald Trump is still thinking that he can end the conflict, then finding a way of ending that conflict is going to involve some of those preconditions and some of the details that have really rather bedeviled the US leader's efforts to broker peace in the conflict up until now."

Earlier, Russia's deputy UN Ambassador Dmitry Polyanskiy said Putin may meet with Trump next week, but he was not aware of any planned meeting between Putin and Zelensky.

"As far as I heard, there are a number of locations, but they agreed to something that they don't want to disclose. The timeline is, I think, next week, but that's again judging from what presidents said themselves," Polyanskiy told reporters about the Putin-Trump meeting.

There has been no summit of US and Russian leaders since Putin and former President Joe Biden met in Geneva in June 2021. (Agencies)