U.S. President Barack Obama announced new sanctions against some Russians on Monday to stop President Vladimir Putin from fomenting the rebellion in eastern Ukraine, but said he was holding broader measures against Russia's economy "in reserve."

On the ground, pro-Moscow rebels showed no sign of curbing their uprising, seizing public buildings in another town in the east. Interfax news agency reported that the mayor of a further major eastern city, Kharkiv, had been shot and was undergoing an operation. It gave no details of the shooting.

Germany demanded Russia act to help secure the release of seven unarmed European military monitors, including four Germans, who have been held by the rebels since Friday.