WASHINGTON (AFP) - The White House said Tuesday that Ukraine faces an "untenable" situation over separatist uprisings in the east and described its military operation against pro-Russia militants as "measured."

White House spokesman Jay Carney preached restraint and "all due caution," but said the Ukrainian government had a "responsibility" to safeguard law and order.

Ukraine's leaders sent tanks and troops towards the flashpoint city of Slavyansk on Tuesday, in response to the seizure of government buildings by pro-Russian gunmen.

Ukrainian forces set up a cement road barrier and begun checking traffic leading to Slavyansk, an economically depressed industrial city of 100,000 that has effectively been under separatist control since Saturday.

On Monday, US President Barack Obama urged his Russian counterpart Vladimir Putin to press pro-Moscow groups to lay down their arms in Ukraine, in a telephone call requested by Moscow.