US President Donald Trump has brushed aside recent Russian remarks questioning the legitimacy of Ukrainian President Volodymyr Zelenskyy, calling them "posturing."

"It doesn’t matter what they say. Everybody’s posturing. It’s all bulls," Trump told reporters on Tuesday following an over three-hour Cabinet meeting at the White House.

He was responding to comments by Russian Foreign Minister Sergey Lavrov, who on Sunday cast doubt on Zelenskyy’s constitutional legitimacy.

"(Russian President Vladimir) Putin is ready to meet with Zelenskyy when the agenda would be ready for a summit," Lavrov told NBC News.

"Legitimacy is another matter."

"We recognise him (Zelenskyy) as the de facto head of the regime, and in this capacity, we are ready to meet with him. But when it comes to signing legal documents...we would need a very clear understanding by everybody that the person who is signing is legitimate and, according to the Ukrainian Constitution, Mr. Zelenskyy is not at the moment," he claimed.

Lavrov also said Putin conveyed Moscow's readiness to continue direct peace talks with Kiev during his latest phone call with Trump, but cautioned that a top-level summit between Putin and Zelenskyy must be "very well prepared."

At his August 15 meeting with Putin in Alaska, Trump suggested the leaders of Russia and Ukraine could meet directly for peace talks aimed at ending the war.

At Tuesday’s Cabinet meeting, Trump’s special envoy Steve Witkoff said discussions on the conflict would take place this week and expressed hope the war could be ended by year’s end.

Earlier Tuesday, Zelenskyy said Kiev was in contact with countries that may host such a summit with the Russian side.