US President Barack Obama has urged his Russian counterpart Vladimir Putin to use his influence to make separatists in eastern Ukraine stand down.
The phone call between the two leaders came as pro-Russian activists continued to occupy buildings in eastern towns.
For his part, Mr Putin rejected accusations of Russian interference, calling the reports “unreliable”.
Meanwhile, Ukraine’s acting President, Olexander Turchynov, has announced the start of an “anti-terrorist operation”.
He told parliament it had begun in the “north of Donetsk Region” on Tuesday morning and was being conducted “stage by stage, in a responsible and weighed manner”.
The extent of the operation was unclear but unconfirmed reports on Russian media, quoting separatists, speak of Ukrainian armour being on the move near the flashpoint towns of Sloviansk and Kramatorsk.
Tanks and armoured personnel carriers could be seen parked 70km (44 miles) from Sloviansk on Monday.
EU foreign ministers say they will expand a list of names targeted by sanctions.
Tension has been steadily rising since Russia annexed the Crimean peninsula, formerly part of Ukraine, last month.
The move, condemned as illegal by Kiev and the West, followed the ousting of Ukraine’s pro-Moscow President Viktor Yanukovych in February.