The UN Security Council has held an emergency session at Russia's request. The meeting followed an ultimatum issued by Kyiv to pro-Russian militants to cease the occupation of government buildings in eastern Ukraine.
The United Nations Security Council convened in New York City late on Sunday as tensions between Ukraine and Russia threatened to spiral out of control.
"There has already been bloodshed and a further escalation must be swiftly stopped," Russian ambassador to the UN Vitaly Churkin told the council on Sunday.
"The international community must ask Kyiv to stop and implement all its commitments," he said, adding that Kyiv must engage in dialogue.
On Sunday, Ukraine's interim president, Oleksandr Turchynov, threatened a "large-scale anti-terrorist operation" against pro-Russian supporters if they did not vacate government buildings in several eastern Ukrainian cities by 6:00 a.m. on Monday (0400 UTC).
The announcement prompted Russia to request the emergency session at the UN Security Council, saying that the threat from the Ukrainian government signalled it was ready to wage a "war against [its]own people."
US alleges Russia of propoganda
Kyiv believes Russia has orchestrated the unrest in a bid to destabilize the country's east and, possibly, to regain territory lost with the break up of the USSR. It has compared the situation to Moscow's recent actions in Crimea, which led to Black Sea Peninsula's secession from Ukraine.
Some Western leaders have openly condoned this view of the political crisis. On Sunday, US ambassador to the UN, Samantha Power, accused Moscow of destabilizing the eastern European country.
"This is the saddest kind of instability. It is completely man made. It was written and choreographed in and by Russia," Power said.
Earlier on Sunday, she had told American broadcaster ABC that the recent activities by militants in cities such as Slovyansk and Donetsk pointed to Kremlin involvement, saying they had "all the tell-tale signs of what we saw in Crimea."
"It's professional, it's coordinated, there's nothing grassroots-seeming about it. The forces are doing - in each of the six or seven cities that they've been active in – exactly the same thing," she said.
Britain: Russia has amassed 40,000 troops
The United Kingdom's UN ambassador presented information on Sunday night which pointed to Russian military aggression.
"Satellite imagines 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," UK ambassador Mark Lyall Grant said. "Russia is seeking to impose its will on the people of Ukraine using misinformation, intimidation and aggression."
"We want to use this Security Council meeting to expose that but also to warn Russia against using events in eastern Ukraine as a pretext for further military escalation in the region."
NATO Secretary General Anders Fogh Rasmussen has also warned of harsher economic sanctions and isolation in reaction to the troop amassment. The US and EU, for their own parts, have also threatened even tougher bans than the ones already levied in reaction to Russia's actions in Ukraine and Crimea.
kms/av (AP, AFP, Reuters, dpa)