Russian President Vladimir Putin met with top military officials to discuss the "deterioration of the situation" amid fresh fighting on fault lines in eastern Ukraine.
Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic, said the Ukrainian army had launched "all-out war" on rebel positions, the Russian news agency RIA said.
Ukrainian military spokesman Vladyslav Seleznyov denied this, saying the army remained in agreed positions.
"We refute these allegations ... we're strictly fulfilling the Minsk memorandum (on a cease-fire)," he told Reuters by telephone.
Russia is also charging Ukraine with deliberately shelling a school, although The Interpreter reports that Moscow has not provided evidence and public information does not point to Ukraine military positions.
Representatives of the separatist regions earlier put out a joint statement calling for a redrafting of the Minsk deal, which established a cease-fire in exchange for Kiev granting "special status" to eastern territories.
Rebels say Ukraine has violated the deal by seeking to revoke a law that would have granted eastern regions autonomy. Kiev says this was a consequence of Sunday's separatist leadership elections, which it says go against the agreement.
At least 4,000 people have died during the conflict.