Washington DC - US President Donald Trump called Thursday for a month-long unconditional ceasefire between Russia and Ukraine, with any breaches punishable by sanctions.

"Talks with Russia/Ukraine continue. The U.S. calls for, ideally, a 30-day unconditional ceasefire," Trump said on his Truth Social network shortly after speaking to Ukrainian President Volodymyr Zelensky.

"If the ceasefire is not respected, the U.S. and its partners will impose further sanctions."

Trump said that "both countries will be held accountable for respecting the sanctity of these direct negotiations" to halt the conflict that started when Russia invaded Ukraine in 2022.

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The US president said he wanted any ceasefire to then build to a "lasting peace."

"It can all be done very quickly, and I will be available on a moment’s notice if my services are needed."

Trump opened talks with Russia's Vladimir Putin in February in a bid to end the war that he had previously pledged to end within 24 hours of starting his second term.

But he has shown growing impatience, first with Zelensky and more recently with Putin as the fighting has continued.

Trump and other top US officials have stepped up warnings in recent weeks that Washington is prepared to walk away from its role as a broker if there is no progress soon.

Zelensky said on social media that he had told Trump Thursday that Ukraine was ready for talks on the war with Russia "in any format" but insisted that there first had to be a full ceasefire.