ON BOARD AIR FORCE ONE (REUTERS) - The White House on Wednesday said it was appropriate for the Ukraine government to take action to restore law and order and urged it to continue to do so in a measured and responsible way.

White House spokesman Jay Carney told reporters travelling on Air Force One that Kiev had responded with "admirable restraint"to destabilising actions in the country by armed pro-Moscow separatists.

Tensions have continued to build in eastern Ukraine, where pro-Russian militiamen have seized control of official buildings despite government threats of a crackdown.

Separatists flew the Russian flag on armored vehicles taken from the Ukrainian army on Wednesday, humiliating a Kiev government operation to recapture eastern towns controlled by pro-Moscow partisans.

Talks between Russia, Ukraine, the United States and the European Union are scheduled for Thursday. Mr Carney said the United States is ready to impose a fourth round of sanctions against Russia, but said the negotiations would have a bearing on US actions.

"We have additional sanctions prepared and we'll impose them as appropriate," Mr Carney said. "We are going to participate in that meeting with an eye toward evaluating whether or not Russia is serious about de-escalating."

He declined to provide details about the sanctions, but said President Barack Obama would have a range of options that would be commensurate with Russian actions in Ukraine. "As Russia escalates, the costs will escalate," Mr Carney said.