Ukraine has given pro-Russian separatists a Monday morning deadline to disarm or face a "full-scale anti-terrorist operation" by its armed forces, raising the risk of a military confrontation with Moscow.
Angered by the death of a state security officer and the wounding of two comrades near the flashpoint eastern city of Slaviansk, acting president Oleksander Turchinov gave rebels occupying state buildings until 6am to lay down their weapons.
"The National Security and Defence Council has decided to launch a full-scale anti-terrorist operation involving the armed forces of Ukraine," Mr Turchinov said in an address to the nation.
He blamed Russia, which recently annexed Ukraine's Crimea region, for being behind the rash of rebellions across Russian-speaking towns in eastern Ukraine.
"We will not allow Russia to repeat the Crimean scenario in the eastern regions of Ukraine," Mr Turchinov said.
Russia's foreign ministry called the planned military operation a "criminal order" and said the West should bring its allies in Ukraine's government under control.
"It is now the West's responsibility to prevent civil war in Ukraine," the ministry said in a statement.
A United Nations Security Council diplomat said on condition of anonymity that the council would meet at 8pm in New York at Russia's request. Another diplomat said negotiations were under way on Ukraine's participation.
Earlier, the American ambassador to the UN, Samantha Power, told on ABC that the latest events in Ukraine bore "the telltale signs of Moscow's involvement".
"The president has made clear that, depending on Russian behaviour, sectoral sanctions in energy, banking, mining could be on the table, and there's a lot in between," she added.
With East-West relations in crisis, NATO described the appearance in eastern Ukraine of men with specialised Russian weapons and identical uniforms without insignia - as previously worn by Moscow's troops when they seized Crimea - as a "grave development".
Ukraine has repeatedly said the rebellions are inspired and directed by the Kremlin. But action to dislodge the armed militants risks tipping the stand-off into a new, dangerous phase as Moscow has warned it will protect the region's Russian-speakers if they come under attack.
One Ukrainian state security officer was killed and five were wounded on the government side in Sunday's operation in Slaviansk, interior minister Arsen Avakov said.
"There were dead and wounded on both sides," he wrote on his Facebook page.
The Russian news agency RIA reported that one pro-Moscow activist was killed in Slaviansk in clashes with forces loyal to the Kiev government.
"On our side, another two were injured,"RIA quoted pro-Russian militant Nikolai Solntsev as adding.
Russian TV broadcast grainy footage of what it said was the body of the militant. The images, which Reuters could not verify independently, showed a man in black clothes, slumped against the door of a car, with a pool of blood between his legs. A rifle lay next to him.