Kiev: 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 Turchynov gave rebels occupying state buildings until 6am GMT to lay down their weapons.
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Kiev gives rebels deadline to disarm
WARNING GRAPHIC IMAGES: violence escalates in Ukraine, as the government launches an “anti-terrorist operation” set to target pro-Russian separatists.
"The National Security and Defence Council has decided to launch a full-scale anti-terrorist operation involving the armed forces of Ukraine," Mr Turchynov said in an address to the nation.
He blamed Russia, which annexed Ukraine's Crimea region when Moscow-backed former president Viktor Yanukovych fled after months of pro-Western protests, 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 Turchynov 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 the council would meet in New York at 8pm, local time, at Russia's request. Another diplomat said negotiations were under way on Ukraine's participation.
Earlier, United States ambassador to the UN Samantha Power said the latest events in Ukraine bore "the telltale signs of Moscow's involvement".
"The [US] 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 told the ABC network in the US.
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 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.
The Russian news agency RIA reported that one pro-Moscow activist was killed in Slaviansk in clashes with forces loyal to the Kiev government.
The separatists are holed up in the local headquarters of the police and of the state security service, while others have erected road blocks around Slaviansk, which lies about 150 kilometres from the Russian border. However, details of the fighting remain sketchy. A statement from the administration of the eastern Donetsk region indicated the security officer may have been killed between Slaviansk and the nearby town of Artemivsk. It said nine were wounded.
Kiev accuses the Kremlin of trying to undermine the legitimacy of presidential elections on May 25 that aim to set Ukraine back onto a normal path after months of turmoil.
However, Russian Foreign Minister Sergei Lavrov said Kiev was "demonstrating its inability to take responsibility for the fate of the country" and warned that any use of force against Russian speakers "would undermine the potential for co-operation", including talks due to be held on Thursday between Russia, Ukraine, the United States and the European Union.
In Kramatorsk, about 15 kilometres south of Slaviansk, gunmen seized the police headquarters after a shootout with police, a witness said. The attackers were a well-organised unit of over 20 men, wearing matching military fatigues and carrying automatic weapons, who had arrived by bus. Video footage showed the men taking orders from a commander. Their identity was unclear.
Their level of discipline and equipment was in contrast to the groups who have occupied buildings so far in Ukraine. They have been mostly civilians formed into informal militias with mismatched uniforms.
NATO Secretary-General Anders Fogh Rasmussen expressed concern about similarities in some of the rebels' appearance to that of the Russian troops who seized control in Crimea.
Calling on Russia to pull back its large number of troops, including special forces, from the area around Ukraine's border, he said in a statement: "Any further Russian military interference, under any pretext, will only deepen Russia's international isolation."