Ukraine accused Russian army officers on Monday of fighting alongside separatists in the east of the country and said Moscow was once more building up its troops on the joint border.
A missile that downed a Ukrainian transport plane carrying eight people near the border was probably fired from Russia, Ukrainian officials said.
President Petro Poroshenko held an emergency meeting of his security chiefs after a weekend of Ukrainian airstrikes on rebel positions near the border with Russia and charges by Moscow that Kiev killed a Russian man with a cross-border shell.