Towns in eastern Ukraine are bracing for military action from government forces as a deadline passed for pro-Russian separatists to disarm and end their occupation of state buildings or face a major "anti-terrorist" operation.
As the 9am deadline issued by authorities in Kiev expired on Monday, Reuters news agency reported that there was no outward sign the rebels were complying with the ultimatum in the city of Slovyansk.
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Angered by the death of a state security officer and the wounding of two comrades near Slovyansk, acting president Oleksander Turchinov warned rebels on Sunday that a full-scale security operation that would include the army, would be unleashed unless they met the deadline.
Turchinov and other leaders blame Russia, which annexed Ukraine's Crimea region when Moscow-backed former president Viktor Yanukovich fled after months of pro-Western protests, for inspiring and organising a rash of rebellion in Slovyansk and other Russian-speaking towns in eastern Ukraine.
"We will not allow Russia to repeat the Crimean scenario in the eastern regions of Ukraine," Turchinov said on Sunday night.
The crisis has brought relations between Russia and Western countries to their lowest point since the end of the Cold War in 1991, and also carries a risk of unleashing a "gas war" which could disrupt energy supplies across Europe.
Use of force by Kiev's pro-Europe authorities could trigger a fresh confrontation from Russia. Russia's foreign ministry called the planned military operation a "criminal order" and said Western countries should bring their allies in Ukraine's government under control.
The United Nations Security Council held an emergency session on Sunday night, and the US warned that it was likely to impose further sanctions on the Kremlin if the escalation in eastern Ukraine continues.
Rebels maintain barricades
In Slovyansk as of 9am local time on Monday, a Russian flag still flew over police headquarters, one of two buildings taken over by the separatists, while masked men continued to man barricades of sandbags in front.
Even as the deadline passed, a truck appeared bringing more tyres to heap on top of the barricades to reinforce them.
There was tension in the air as people tried to go about their normal business, though school and colleges have been closed and parents advised to keep their children indoors.
European Union foreign ministers were due to meet in Strasbourg on Monday to discuss Ukraine.