EU foreign policy chief Kaja Kallas told AFP Tuesday that the US has not used up its "tools" to pressure Russia, as she accused Moscow of playing for time with its Easter truce in Ukraine.
"They didn't keep to the truce. I mean, it's clear that Russia is playing all these games and stalling and does not want peace," Kallas said in an interview.
"They think that time is on their side, so they're not showing any good signs, or any good will," she said.
President Donald Trump threatened last week to walk away from efforts to halt the Ukraine war if there was not quick progress from Moscow and Kyiv.
The US leader later said he hoped for a Russia-Ukraine peace deal "this week", promising "big business with the United States" for both combatants if a ceasefire is signed.
Kallas said Russia's announcement of a 30-hour truce over the weekend was a ploy aimed at stopping Trump "losing patience" with the Kremlin.
The EU's top diplomat said that Kyiv and its European backers were hoping Washington would take a tougher line with Moscow before abandoning its peace push.
"They have tools in their hands to use to pressure Russia. They haven't used those tools," Kallas said.
"If they are now walking away without using the tools that they have actually in their hands, then my big question mark is, why? Why aren't they using the tools to end this war?"
Kallas said that she feared Trump might wash his hands of the war in Ukraine -- but still look to pursue "deals and business as usual" with Moscow regardless.
Ukraine's allies are set to meet in London on Wednesday to discuss US proposals for ending the war.
Kallas warned that it would be a mistake for the US to consider recognising the occupied Ukrainian region of Crimea as Russian territory as part of any deal.
"Then Russia clearly gets what they want," she said.
She said that the EU for its side would never recognise the peninsula -- seized by Moscow in 2014 -- as Russian.
"Crimea is Ukraine," Kallas said.
"It means a lot for the ones who are occupied that others don't recognise this as Russian."
The EU has so far been excluded from the US-led peace efforts, but Washington concedes the bloc will have to be involved at some stage given the vast sanctions it has imposed on Russsia.
The Kremlin is desperate to get the economic punishment lifted as part of any negotiations.
But Kallas warned that the EU should not ease any of its sanctions against Moscow before Russia has proved it is living up to any final peace deal.
"We need to see credible proof, also from Russian side, that they are keeping to this agreement before we make the steps," Kallas said.
"I think we have to see this through, really."