The UK accused Moscow of "breathtaking" hypocrisy after bloody clashes between pro-Russian militia and Ukraine's army marked a sharp escalation in the crisis.
At an emergency session of the United Nations Security Council (UNSC), the UK's ambassador SIr Mark Lyall Grant said Russia "funded, equipped and directed" some of those involved in the insurgency.
A clash in the southern city of Odessa left at least 31 dead and the Ukrainian military was involved in operations against armed pro-Russian groups in the country's east.
The UNSC tonight again failed to take action on the growing crisis in Ukraine, with Western powers dismissing Russia's apparent show of indignation as its ambassador Vitaly Churkin demanded a "swift halt of all violence".
Sir Mark said: "There is no Council member sitting around this table that would allow its towns to be overrun by armed militants.
"There is none of us that would abrogate responsibility for the protection of citizens on our own territory who are being intimidated and brutalised by heavily-armed groups backed by a neighbouring country.
"Proportionate is not the same as passive.
"The scale of Russian hypocrisy is breathtaking. Russia stoutly supports and indeed arms the most repressive regimes in the world, notably Syria, a regime which brutally represses dissent without any sense of restraint or concern for the protection of civilians.
"Russia's synthetic indignation of Ukraine's proportionate and measured actions convinces no one."
Ukraine's a cting president Oleksandr Turchynov claimed that "many" insurgents had been killed or wounded in the military offensive launched against opposition forces in Sloyvansk.
But insurgents succeeded in downing two military helicopters, reportedly using surface-to-air missiles.
Sir Mark said it was "simply not credible" for Moscow to claim that the pro-Russian forces were "peaceful activists" given the apparent use of man-portable air defence systems.
"The use of such sophisticated weaponry against Ukrainian forces reaffirms our assessment that the armed groups in east Ukraine include professionals funded, equipped and directed by Russia."
The Kremlin said the move by Kiev's interim government effectively killed the Geneva pact reached last month which was aimed at cooling the unrest, but pledged to continue efforts to try to calm the tensions.
Sir Mark said: "We urge Russia to throw its full weight behind the April 17 agreement and to rein in the militant armed groups which it supports and which are responsible for the current crisis."
Foreign Secretary William Hague, who will visit Ukraine next week, said: "I am extremely concerned by reports of the activities of armed pro-Russian separatists in eastern Ukraine, including the detention of hostages, intimidation of the media and the reported shooting down of Ukrainian military helicopters this morning.
"There can be no doubt that these incidents, perpetrated by well-trained groups using sophisticated military technology, are intended to provoke further instability.
"These provocations are destroying all efforts to reduce tensions and are a serious threat to the country.
"Ukraine has shown admirable restraint since the beginning of this crisis in the face of extreme provocation on its own national territory.
"Now more than ever it is important that all parties, including the Russian Federation, abide by the commitments they made in Geneva on 17 April.
"I urge the Russian authorities to act to reduce tensions and to call upon separatist groups to lay down their arms."
Unlike eastern Ukraine, Odessa had remained largely untroubled since the February toppling of president Viktor Yanukovych, who fled to Russia.
But a clash erupted between pro-Russians and government supporters in the key port on the Black Sea coast resulting in a deadly fire in a trade union building.