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.
The goal of the "anti-terrorist operation" is to protect Ukraine's citizens and stop criminal activity and attempts to tear the country apart, said Turchynov.
"An anti-terrorist operation began in the north of Donetsk Oblast. It will be conducted step-by-step, responsibly, deliberately. The goal of these actions, I want to underline, is to defend the citizens of Ukraine," Turchynov told the Ukrainian parliament.
"I call on those who want to show their heroism – join the National Guard or the Ukrainian army or police units. But smashing the windows of the Verkhovna Rada is not a heroism at all."
The reference is to a group of pro-Russian activists who gathered near the parliament in Kiev demanding the resignation of interior minister Arsen Avakov.
"People are asking for help now, and we need to do everything to defend them." Turchynov continued.
Clashes with pro-Russian gunmen have left at least one Ukrainian officer dead.
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.
"The president emphasised that all irregular forces in the country need to lay down their arms, and he urged president Putin to use his influence with these armed, pro-Russian groups to convince them to depart the buildings they have seized," the White House said in a statement.
"The president noted Russia's growing political and economic isolation as a result of its actions in Ukraine and made clear that the costs Russia already has incurred will increase if those actions persist," the White House said.
"[He] said that while he continues to believe that a diplomatic solution is still possible, it cannot succeed in an environment of Russian military intimidation on Ukraine's borders, armed provocation within Ukraine, and escalatory rhetoric by Kremlin officials."