US Secretary of State John Kerry has called on Russia to disarm separatists in Ukraine within "the next hours" as the EU prepared to discuss deeper sanctions against Moscow.
Washington and other Western powers have stepped up pressure on Russia to take concrete action to defuse the conflict in eastern Ukraine, where a ceasefire between Russian-speaking rebels and government forces has appeared to be crumbling.
Mr Kerry said: "We are in full agreement that it is critical for Russia to show in the next hours, literally, that they're moving to help disarm the separatists, to encourage them to disarm, to call on them to lay down their weapons and to begin to become part of a legitimate process."
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He added that EU leaders would discuss possible sanctions on Russia at their summit in Belgium today.
Washington has said it also has new sanctions ready to go but Mr Kerry said the US would prefer not to be in "sanctions mode" and wanted Russia to take action without pressure.
Moscow denies Western accusations it has allowed fighters to cross into Ukraine along with heavy weapons to confront Ukrainian government forces.
The proposed next round of US and EU sanctions would target Russia's financial, defence and high-tech industries, said US officials.
Mr Kerry is on a tour of capitals in the Middle East and Europe to discuss Ukraine, as well as the threat to stability in the Middle East from conflicts in Iraq and Syria.
Earlier, Ukrainian separatist rebels agreed to take part in further peace talks to end the conflict.
Ukrainian President Petro Poroshenko said he had just heard of the rebels' readiness to meet again with the so-called "contact group" which includes former Ukrainian President Leonid Kuchma, Moscow's envoy to Kiev and a high-ranking official from the OSCE security and rights watchdog.
But Ukrainian media quoted him as hinting heavily that there would be no extension of a government ceasefire, which officially ends at 10pm today.