Ukraine's acting president Oleksandr Turchynov has announced the launch of a military operation in the eastern Donetsk region to crack down on pro-Russian protesters who have seized government buildings.

Turchynov said the goal of the anti-terrorist operation is to protect Ukraine's citizens from criminal activity and attempts to tear the country apart.

"Today in the morning an anti-terrorist operation has started in the east of Donetsk region. But it will be carried out stage by stage, responsibly and cautiously. The aim of these actions - I underline it one more time - is to protect the citizens of Ukraine, to stop terror, and to stop criminality, stop attempts to tear Ukraine to pieces," he said.

The development came after US president Barack Obama told Vladimir Putin that Moscow would face another round of sanctions for its actions in Ukraine.

Speaking by phone, Obama also warned his counterpart that he should use his leverage among pro-Russian protesters in Ukraine to tell them to stand down.