NEW YORK, April 13 - 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, Reuters reported.

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 Oleksandr Turchynov gave rebels occupying state buildings until 9:00 AM 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," 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," Turchynov 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 Nalyvaychenko 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.

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 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.

In Kramatorsk, about 15 km south of Slaviansk, gunmen seized the police headquarters after a shootout with police, a Reuters witness said.

The attackers were a well-organized unit of more than 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 which have occupied buildings so far in Ukraine. They have been mostly civilians formed into informal militias with mismatched uniforms.

In Washington, the U.S. State Department said pro-Russian militants seizing government buildings in six cities in eastern Ukraine on Saturday was an orchestrated operation reminiscent of those conducted in Crimea before it was annexed by Russia.

"Many of the militants were outfitted in bullet-proof vests and camouflage uniforms with insignia removed and carrying Russian-origin weapons," it said in a note entitled "Evidence of Russian Support for Destabilization of Ukraine." (rt/ez)