Russia accused NATO of whipping up dangerous tensions near its borders and encouraging Ukraine to use force against pro-Russian separatists.
At a tense meeting in Brussels, the alliance urged Moscow to stop arming the rebels. In Luhansk, at least two people were killed and several injured in an explosion which separatists controlling the eastern city said was the result of an air strike by the Ukrainian military.
This was denied by Kiev, which said it was caused by separatists who had launched a heat-seeking rocket at a Ukrainian plane but the missile had zeroed in instead on the rebel-occupied regional administration building.
Ukraine and its Western allies accuse Moscow of fuelling the pro-Russian uprising that threatens to break up the former Soviet republic of 45 million people.
Russia denies orchestrating the unrest, and says Ukraine's attempts to end it by military force are making the situation worse.
In an a sign of rapprochement, Russia's and Ukraine's gas companies agreed to consider a plan for Kiev to pay off its outstanding gas debts, including a proposal that should ensure security of supply until June 2015, European Union Energy Commissioner Guenther Oettinger said.
Earlier, in a conciliatory signal, Russia's Gazprom had given Ukraine until June 9 to resolve the two countries' long-running row over gas pricing, postponing a threat to cut off supplies as early as Tuesday.