Pro-Russian insurgents in Ukraine said they plan to hold a referendum on autonomy for eastern regions on Sunday, although no visible preparations for the vote have yet been seen.
Russia has put the blame for the unrest squarely on the interim government in Kiev. During a Tuesday meeting in Vienna with the Council of Europe, Russian Foreign Minister Sergey Lavrov demanded that the Ukrainian government end its armed assaults on rebel strongholds. He said he was open to another round of international talks to ease the crisis, but only if pro-Russia rebels were included.
"Those who protest ... want their voices heard," he told reporters. "They want to have an equal voice when it comes to deciding the fate of their own country."
His Ukrainian counterpart rejected the proposal, saying the Kiev government already represents all the people of Ukraine.
Ukrainian authorities have blamed Moscow for fomenting the unrest in the east, saying it's an attempt to derail Ukraine's May 25 presidential election. Lavrov repeated Moscow's claims that violence in Ukraine proved the country was unready for a vote, and that a constitution allowing for greater federalization should come before a presidential election.
The Obama administration Tuesday denounced the referendum as illegal.
Secretary of State John Kerry said the referendum would be "bogus" and not recognized by the West.
Last month Moscow supported an independence referendum in the Ukrainian peninsula of Crimea, which led to its annexation by Russia. Leaders of the anti-government movement say they plan to hold a referendum on autonomy in the areas of Donetsk and Luhansk, although no preparations for the vote have yet been seen.
"We flatly reject this illegal effort to further divide Ukraine," Kerry told reporters after meeting at the State Department with EU foreign policy chief Catherine Ashton. "It's pursuit will create even more problems in the effort to try to de-escalate the situation."
He added: "This is really the Crimea playbook all over again, and no civilized nation is going to recognize the results of such a bogus effort."
Although Russia has de facto control over Crimea, few nations recognize the move. The United States and Europe have hit Russia with sanctions over the annexation and Moscow's continued destabilizing actions in mainly Russian-speaking east Ukraine.
In a nod to the West's refusal to recognize Crimea as part of Russia, State Department spokeswoman Jen Psaki on Tuesday responded to a reporter's question about a potential visit to the region by Russian President Vladimir Putin by saying: "I hope he enjoys his visit to Ukraine."
U.S. officials have said all Ukrainians should vote on their future governance in nationwide elections set for May 25.
Meanwhile, both Kerry and Ashton lambasted Russia for failing to abide by an agreement to de-escalate tensions in the east and threatened that any move by Moscow to annex additional parts of Ukraine would be met by additional sanctions.
Such sanctions, they said, would go beyond penalties already imposed on Russian officials and executives close to Putin and apply to entire sectors of the Russian economy, such as defense, finance and energy.
In Ukraine on Tuesday, Ukrainian troops tightened a security cordon around a major insurgent-held eastern city but pro-Russia militia acted with impunity elsewhere in the turbulent region bordering Russia, surrounding a major Interior Ministry base.
Thirty pro-Russia insurgents and four government troops were killed Monday in operations to expunge anti-government forces around the city of Slovyansk, Ukraine's interior minister said Tuesday. Rebels said 10 people -- fighters and civilians -- were killed by Ukrainian troops during clashes Monday. They would not elaborate and there was no immediate way to reconcile the figures.
Gunbattles on Monday around the city of 125,000 were the interim government's most ambitious effort to date to quell weeks of unrest in Ukraine east.
The Associated Press contributed to this report.