The head of the Organisation for Security and Cooperation in Europe, Swiss President Didier Burkhalter, was to meet Russian leader Vladimir Putin after Moscow quashed a German-led plan for new peace talks.
And Britain's Foreign Secretary William Hague held talks in Kiev, as the Western-backed government there stepped up its military offensive to seize back a string of eastern towns and cities under the control of pro-Russian gunmen.
On the ground, local media reported that Ukrainian security forces had "liberated" the town hall in the port city of Mariupol near the Russian border.
Authorities there claim to have killed more than 30 rebels in a relentless assault on the city. But they have also lost at least nine personnel and three helicopter gunships in the battle with well-armed separatists.
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The Russian flag that used to fly over the town hall has been taken down and rebels said they could either stay in the barricaded building they have held for several weeks or abandon it as the situation demands.
Germany too has said it fears an all-out military conflict, as Putin weighs whether to launch an invasion with the estimated 40,000 troops he has stationed on the border.
With relations between Moscow and the West plumbing depths not seen since the end of the Cold War, the United States and Russia continued to trade barbs as the clock ticked towards an independence referendum planned in eastern Ukraine on May 11.