The United Nations Security Council held an emergency session on Sunday night to discuss the escalating crisis in Ukraine, just hours before a deadline by Kiev for pro-Russian separatists to disarm by Monday morning or face a "full-scale anti-terrorist operation" by its armed forces.
The Council began meeting at 8 p.m. at Russia's request after Moscow called Kiev's plans to mobilize the army to put down a rebellion by pro-Russian militants in eastern Ukraine "criminal."
Britain's U.N. ambassador said Russia had massed tens of thousands of well-equipped troops near the Ukrainian border in addition to the 25,000 troops it recently moved into Crimea, which Moscow seized last month.
"Satellite images show that there are between 35,000 and 40,000 Russian troops in the vicinity of the border with Ukraine equipped with combat aircraft, tanks, artillery and logistical support units," Ambassador Mark Lyall Grant said.
"This is in addition to the 25,000 Russia troops based illegally in Crimea," Lyall Grant added in his speech during the U.N. emergency meeting.
Angered by the death of a state security officer and the wounding of two comrades near the flashpoint eastern city of Slaviansk, Ukrainian acting president Oleksander Turchinov gave rebels occupying state buildings until 0600 GMT (2 a.m. EDT) to lay down their weapons.
"The National Security and Defense Council has decided to launch a full-scale anti-terrorist operation involving the armed forces of Ukraine," Turchinov said in an address to the nation.
He blamed Russia, which annexed Ukraine's Crimea region when Moscow-backed former president Viktor Yanukovich 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," Turchinov said.
The deadline and the standoff with Russian troops at the border have raised fears of a military confrontation with Moscow.
The head of Ukraine's state security service (SBU) said government forces would respond ruthlessly if pro-Russian separatists opened fire.
"If they open fire, we will annihilate them. There should be no doubt about this," Valentyn Nalyvaichenko said in a televised interview.
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.
The 15-nation council has held numerous emergency meetings on Ukraine but has been incapable of taking any concrete action because of Russia's sharp disagreements with the United States and Europe.
Earlier, the American ambassador to the U.N., Samantha Power, said on ABC's "This Week" that the United States was prepared to step up sanctions against Moscow if pro-Russian military actions in eastern Ukraine continued.
"The president has made clear that, depending on Russian behavior, sectoral sanctions in energy, banking, mining could be on the table, and there's a lot in between," she added.
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.
WELL ORGANISED ATTACKERS
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, about 150 km (90 miles) from the Russian border.
Kiev accuses the Kremlin of trying to undermine the legitimacy of presidential elections on May 25 that aim to set Ukraine back on 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 cooperation", including talks due to be held on Thursday between Russia, Ukraine, the United States and the European Union.