EU leaders are to meet in Brussels to consider fresh sanctions against Russia following accusations that its troops are fighting inside Ukraine.
Russia denies that its regular forces are backing a rebel offensive in eastern Ukraine.
But ahead of the summit, EU officials said a clear message must be sent to Russia to de-escalate the crisis.
In Ukraine, a number of government troops remain encircled by the rebels in the Donetsk region.
Reports say the pro-Russian fighters have issued an ultimatum to the Ukrainian troops near the town of Ilovaisk to surrender by Saturday morning.
