Towns in eastern Ukraine on Monday braced for military action from government forces after Kiev gave pro-Russian separatists a 9 a.m. (0600 GMT) deadline to disarm and end their occupation of state buildings or face a major "anti-terrorist" operation.
As the deadline passed,in the flashpoint city of Slaviansk, where armed men had seized two government buildings,there was no outward sign the rebels were complying with the ultimatum.
Angered by the death of a state security officer and the wounding of two comrades near Slaviansk, acting president Oleksander Turchinov warned rebels on Sunday that a full-scale security operation, including the army, would be unleashed unless they met the deadline.
Oleksandr Valentynovych Turchynov ,a Ukrainian politician, 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 rebellions in Slaviansk 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 the West to their lowest point since the end of the Cold War in 1991, and also carry 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 the West should bring its allies in Ukraine's government under control.
The United Nations Security Council held an emergency session on Sunday night, and the United States warned that it was likely to impose further sanctions on the Kremlin if the escalation in eastern Ukraine continues.
In Slaviansk, a town of about 120,000 people where separatists are occupying a three-storey police headquarters and the offices of the state security service, there was tension in the air as people tried to go about their normal daily business.
In front of the police headquarters occupied by the separatists a group of about 40 people, who are there in solidarity with the rebels, were warming themselves by blazing fires from oil barrels.
Barricades closing off entry to the building were still manned and there were no external signs of any surrender of arms.
School and colleges have been closed and parents advised to keep their children indoors.
Alexei Myzenko, a 38-year-old bank teller, was at work as usual, but he said he and his wife had told their son, who is at university in the eastern town of Kharkiv, not to attend lectures on Monday.
"We didn't want anything to happen to him," said Myzenko. "Of course, some people are afraid. But they are still lining up to get their pensions," he said.
Myzenko said his wife, who is a teacher, had been called by the town administration to tell her that school was cancelled until further notice.
Iryna Zemlyanskaya, 62, who works as a pharmacist, said: "I am going to work. They've promised to use force so many times and have not done a single thing. No-one's even afraid anymore."