The United Nations security council held an emergency session on Sunday night to discuss the escalating crisis in Ukraine as the war of words between its western allies and Russia continued.

Just hours before a deadline by Ukraine for pro-Russian separatists in eastern cities to disarm by Monday morning or face all-out attack, the security council convened at Russia's request. Moscow called Kiev's plans to mobilise the army "criminal".

Russian ambassador to the UN, Vitaly Churkin, denied western and Ukrainian claims that Moscow was behind the violence, and told the meeting that Ukraine has been using radical neo-Nazi forces to destabilise its eastern region.



"It is the west that will determine the opportunity to avoid civil war in Ukraine. Some people, including in this chamber, do not want to see the real reasons for what is happening in Ukraine and are constantly seeing the hand of Moscow in what is going on," Churkin said. "Enough. That is enough."

Churkin's comments were a direct rebuke to US and its allies which continued on Sunday to link the Kremlin to the unrest in eastern Ukraine.

His US counterpart Samantha Power told the meeting: "These armed units ... raised Russian and separatist flags over seized buildings and have called referendums and union with Russia. We know who is behind this."

Britain's UN 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 effectively annexed 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," Mark Lyall Grant said.

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 (0200 EST) on Monday to lay down their weapons.

"The national security and defence 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.

Earlier, 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.

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.

Relations between Russia and the West are at their worst since the Cold War, due to the crisis that began when Moscow-backed Yanukovich was pushed out by popular protests in February.

Moscow then annexed Crimea from Ukraine, saying the Russian population there was under threat. Some Western governments believe the Kremlin is preparing a similar scenario for eastern Ukraine, something Moscow has strenuously denied.

In Kramatorsk, about 15 km (9 miles) south of Slaviansk, gunmen seized the police headquarters after a shootout with police, a Reuters witness said.

The attackers were a well-organised 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.

Washington and Moscow have maintained regular dialogue throughout the crisis and on Saturday John Kerry, the US secretary of state, spoke by telephone to Russia's foreign minister, Sergei Lavrov.



But on Sunday night the US state department released a statement saying that the pro-Russian operations over the weekend were highly organised and "planned in advance" with militants "outfitted in bullet-proof vests and camouflage uniforms with insignia removed and carrying Russian-origin weapons".

The actions were "inconsistent with political, grassroots protests" and that the Ukrainian government had evidence that Russian intelligence officers were directly involved in orchestrating the activities.