Brussels, Belgium - US Secretary of State Marco Rubio said Friday that Donald Trump was not "going to fall into the trap of endless negotiations" with Russia over Ukraine, adding it would soon be clear if Moscow was serious about peace.

Rubio was in Brussels for talks with NATO allies, most of whom have nervously eyed the US president's outreach to the Kremlin to press for a deal to end the Ukraine war.

"President Trump is not going to fall into the trap of endless negotiations about negotiations," Rubio told journalists after the meeting.

"We will know soon enough, in a matter of weeks, not months, whether Russia is serious about peace or not."

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Allies are pleading with Trump to stand strong against Moscow as he pushes for a partial ceasefire despite the warring sides trading accusations of ongoing strikes.

While Trump has pressed for a thaw in ties with Moscow, he has also voiced anger at the slow pace of negotiations to reach a ceasefire.

Russian President Vladimir Putin last month rejected a US-Ukrainian proposal for a full and unconditional ceasefire and has made a US-proposed truce in the Black Sea dependent on the West lifting some sanctions.

Patting Ukraine on the shoulder, Rubio said Kyiv had "shown a willingness" to enter into a complete ceasefire and create space for negotiation.

"The Russians and Putin will have to make a decision about whether they're serious for peace or not," he said.

"If it's a delay tactic. The President's not interested in that."