German Chancellor Angela Merkel and French President Francois Hollande are due in Moscow after getting Kiev's tentative backing for a peace plan aimed at ending surging violence in Ukraine.
Ukrainian President Petro Poroshenko said talks with Merkel and Hollande late on Thursday raised "hope for a ceasefire" after the duo jetted into Kiev in the biggest push yet to resolve the 10-month conflict.
The two European leaders are set to meet Russian President Vladimir Putin - who the West sees as the mastermind behind Ukraine's pro-Moscow rebellion - to try to get him to sign up to their peace plan.
The frantic diplomacy to end the worst East-West crisis since the end of the Cold War came as US Secretary of State John Kerry also visited Kiev, with Washington mulling supplying Ukraine with arms to battle pro-Russian rebels.
"President Putin can make the choices that could end this war," Kerry said after meeting Ukrainian leaders, voicing support for the "helpful" Franco-German plan to be put to the Russian leader on Friday.
Hundreds of civilians have been killed over recent weeks in east Ukraine as fighting spiralled after insurgents tore up an earlier truce and pushed into government-held territory.
Before setting off on the surprise diplomatic push, Francois Hollande said in Paris that he and Merkel would "propose a new solution to the conflict based on the territorial integrity of Ukraine".
Few details have emerged of what exactly the new peace proposal contains and there is much disquiet in Kiev after the collapse of previous peace deals.
The plan appears to have come from a back-and-forth between Putin, Merkel and Hollande and, according to a report in German newspaper Sueddeutsche Zeitung, would give separatists greater autonomy and territory they have captured in recent days, although German officials have denied this.
Despite the diplomatic momentum behind the new plan, Ukrainian Prime Minister Arseniy Yatsenyuk insisted Moscow should just stick to a widely flouted truce accord agreed in Minsk last September.
"To have a new deal, not to execute the previous one, seems to me a trap," Yatsenyuk told journalists.
"We urge Russia to implement and execute what was agreed, signed personally by President Putin."
Yatsenyuk warned that the Russian strongman could be seeking to "split the unity between the EU and the US" at a time when the White House is edging closer to starting weapons deliveries to Ukraine.
After his meetings in Kiev, Kerry said that US President Barack Obama would decide "soon" on whether to arm Ukraine, but stressed his preference for a diplomatic solution.
"We are not interested in a proxy war - our objective is to change Russia's behaviour and we'll consider all options that are available to us in co-ordination with our partners that will help us achieve a negotiated solution," Kerry said.
Russia, accused by the West of arming the separatists, warned that any US move to send weapons to Ukraine would cause "colossal damage" to ties, foreign ministry spokesman Alexander Lukashevich said.
Kerry is to meet Russian Foreign Minister Sergei Lavrov at a security conference in Munich on Saturday, ahead of Obama holding a "very important" meeting with Merkel at the White House on Monday, Kerry said.
In a sign of growing international angst over the conflict, NATO on Thursday agreed on a major boost to the alliance's defences near its Russian borders, including six command centres and a quick-reaction spearhead force of 5000 troops.
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