Ukraine's president has sought urgent talks with his Russian counterpart after rebels shot down an army helicopter despite a ceasefire.
Petro Poroshenko said he hoped Angela Merkel, Germany's chancellor, and Francois Hollande, the French president, would join him on a conference call to Russia's Vladimir Putin on Wednesday.
On Tuesday, the death of nine servicemen in the helicopter near Slovyansk, and loss of two other soldiers in attacks by separatist gunmen, prompted Poroshenko to threaten to begin a powerful new military campaign in the industrial east.
William Hague, the British foreign secretary, said on Wednesday that the helicopter incident was hard to reconcile with Putin's backing of the peace plan.
"We have not seen the actions to match that," Hague said as he arrived for a NATO foreign ministers meeting dominated by the Ukraine crisis. No Putin follow-up, "means the case for sanctions gets stronger", he said.
Russia said on Wednesday it hoped Kiev and the international community would take heed of the "positive signals" it was sending over the Ukraine crisis.
"We are counting on the positive signals that the Russian president is now sending being heard across the world and, above all, in Ukraine," Grigory Karasin, Russia's deputy foreign minister, told Russian news agencies.
On Wednesday, on Putin's request, the upper house of Russian parliament cancelled a resolution allowing the use of military in Ukraine.
The AFP news agency on Wednesday reported new shelling in Slovyansk by Ukrainian forces, who have effectively surrounded the city of nearly 120,000.
Their push was met with extended rounds of anti-aircraft and heavy machine gun fire, the agency reported.