Ukraine was threatened with disintegration Monday as pro-Kremlin militants seized government buildings in the eastern city of Donetsk, declared independence and vowed to vote on joining Russia.
The activists proclaimed the creation of a "people's republic" and appealed to Russian President Vladimir Putin to send a "peacekeeping contingent of the Russian army" to support them.
Ukraine accused Russia of fomenting the unrest and in Washington, the White House responded by calling on Moscow to stop efforts to "destabilise Ukraine" and threatened sanctions.
But Moscow brushed off the accusations and called the latest trouble a sign of Kiev's Western-backed leaders' ineptitude and illegitimacy.
The Cold War-style war of words came as Russian troops remained massed on the Ukrainian border.
Since Russia annexed Crimea, several mainly Russian-speaking eastern regions have seen calls for referendums on joining Russia when Ukraine holds snap presidential polls on May 25.
The political pressure on Kiev's embattled leaders reached boiling point on Sunday when thousands of activists chanting "Russia!" seized administration buildings in Kharkiv and Donetsk as well as the security service headquarters in the eastern region of Lugansk.
The Donetsk activists went one step further on Monday by proclaiming the creation of a sovereign "people's republic".
A video posted on YouTube showed one bearded Russian speaker telling the packed assembly: "Seeking to create a popular, legitimate, sovereign state, I proclaim the creation of the sovereign state of the People's Republic of Donetsk."
Ukraine's Channel 5 television showed an unidentified speaker asking Putin to send a "peacekeeping contingent of the Russian army" to Donetsk to help the region stand up to Kiev's rule.
- 'Stop pointing the finger' -
Ukraine's acting president, Oleksandr Turchynov, accused Russian "special services" of being behind the uprising and ordered extra security personnel to the restless region.
"These actions are meant to destabilise the country, overthrow the Ukrainian government, torpedo the elections and tear our country to pieces," Turchynov said in a nationally televised address.
The Russian foreign ministry responded with a toughly worded statement telling Kiev to "stop pointing the finger at Russia, blaming it for all the problems in today's Ukraine".
But the White House put the onus back on Moscow by describing the latest developments "as the result of increasing Russian pressure on Ukraine".
"We call on President Putin and his government to cease efforts to destabilise Ukraine," White House spokesman Jay Carney said.
Washington's concern underscores the trouble Kiev may have in bringing order to Ukraine's eastern industrial heartland -- a region with ancient cultural and trade ties to Russia.
The Donetsk administration building on Monday remained surrounded by about 2,000 Russia supporters -- some of them armed.
Militants also seized the Donetsk security service headquarters while rival rallies gripped the heart of the nearby city of Kharkiv after its own administration building was occupied briefly overnight.
- Seeking to 'dismember Ukraine' -
Nevertheless, Washington held open the door to diplomacy, with John Kerry discussing in a call with his Russian counterpart the possibility of talks between the United States, Russia, Ukraine and the European Union.
On Tuesday French President Francois Hollande was to meet with NATO chief Anders Fogh Rasmussen to discuss the crisis. Rasmussen last week voiced NATO's "grave concern" over a "very massive Russian military buildup along the Ukrainian border".
Also on Tuesday, a Ukrainian delegation, led by the country's energy minister, Yuriy Prodan, will meet with EU officials in Brussels to discuss providing energy to Ukraine, with the country under pressure from Russia's decision to increase gas prices.
Much of the blame in Kiev was levelled directly at Putin -- a sign of how relations between the two neighbours have plunged in the past few months.
Ukrainian Prime Minister Arseniy Yatsenyuk said Russia had helped orchestrate the occupations in order to find an excuse for a full invasion that would punish Kiev for its decision to seek a political and economic alliance with the West.
"This scenario is written by the Russian Federation and its only purpose is to dismember Ukraine," Yatsenyuk told a government meeting in Kiev.
Moscow is now lobbying for Ukraine to be transformed into a federation that allows eastern regions to adopt Russian as a second state language and overrule some decisions coming from Kiev.
But Washington and its EU allies fear Russia is using the federation idea as an excuse to further splinter Ukraine by granting the Kremlin veto powers over Kiev's regional policies.
The new Kiev government approved a draft reform plan last week that would grant more powers to the regions in line with Western wishes but stopped well short of creating the federation sought by Russia.
And Yatsenyuk on Monday called federalisation a dangerous idea aimed at ruining Ukraine.
"Any call toward federalisation is an attempt to destroy the Ukrainian state," he said.