EU High Representative for Foreign Affairs and Security Policy Kaja Kallas told AFP on Tuesday that the US had not used its "tools" to pressure Russia, and accused Moscow of trying to buy time with the Easter ceasefire in Ukraine.
"They didn't stick to the ceasefire. It's clear that Russia is playing all these games to delay and that they don't really want peace. They think time is on their side, so they're not showing any good signs, no goodwill," Kallas said.
US President Donald Trump threatened last week to abandon efforts to stop the war in Ukraine if there is no rapid progress in a deal between Moscow and Kiev.
He said he hoped for a peace agreement between Russia and Ukraine this week, promising a "big deal with the US" for both countries if a ceasefire is signed.
Kalas said Russia's announcement of a 30-hour ceasefire over the weekend was a ploy aimed at preventing Trump from "losing patience" with the Kremlin.
She added that Kiev and its European supporters hope that Washington will take a tougher stance towards Moscow before giving up on the peace effort.
"They have the tools in their hands to put pressure on Russia, they haven't used them. If they're leaving now without using those tools, then my big question mark is, why? Why aren't they using them to actually end this war?" said Kalas.
She also said that she fears that Trump might "wash his hands" of the war in Ukraine, but that regardless, he still wants to continue to maintain agreements and business "as usual" with Moscow.
Kalas warned that it would be a mistake for the US to consider recognizing the occupied Ukrainian region of Crimea as Russian territory within any agreement.
She stressed that the EU, for its part, will never recognize the peninsula that Moscow seized in 2014:
"Crimea is Ukraine. It means a lot to those who are occupied that others do not consider it Russian."
The EU has so far been excluded from the US-led peace effort, but Washington acknowledges that the bloc will have to be involved at some stage, given the sanctions it has imposed on Russia.
Kalas believes that the EU should not ease any of the sanctions on Moscow before Russia proves that it is fulfilling any final peace agreement.
We need to see credible evidence from the Russian side that they are adhering to the agreement before we take such steps, the European official stressed.
Ukraine's allies will meet in London on Wednesday to discuss US proposals to end the war.
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