The Obama administration and Western allies were scrambling to ease another crisis in Ukraine as pro-Russian protesters seized or blocked government buildings in what U.S. officials said was clearly Moscow's doing.
U.S. Ambassador to the U.N. Samantha Power sparred late Sunday with her Russian counterpart during an emergency session of the U.N. Security Council.
"We are being bombarded by disinformation and propaganda while the Ukrainians are being confronted by violence and incitement," she said.
In a scenario strikingly similar to the fast-paced developments in Crimea in February and March, Western nations are accusing Russia of fomenting protests in Ukraine while Russia claims the protests are an organic response to Ukraine's failure to consider "the legitimate needs and interests" of the Russian-speaking population.
At the Security Council meeting, Russia's U.N. ambassador said officials were concerned Ukraine intends to use military force against demonstrators in southeastern Ukraine.
This type of rhetoric has raised concerns that Moscow is replicating what happened in Crimea, the Russian-speaking region annexed last month amid claims that the government was trying to protect ethnic Russians.
"We feel very strongly that the pattern of activities bears striking similarities to the situation in Crimea ahead of the illegal occupation and reported annexation of that part of Ukraine," State Department spokeswoman Jen Psaki said Monday.
Power, speaking to ABC's "This Week," said the latest unrest bears the "telltale signs of Moscow involvement" and that tougher penalties on Russia could follow if the aggression continues. Treasury Secretary Jacob Lew also said Monday that the U.S. and its allies are "fully prepared to impose additional significant sanctions" on Russia if it continues to stoke tensions with support for a "concerted campaign by armed militants" in eastern Ukraine.
In a startling development, the Pentagon reported Monday that a Russian jet made several close passes by a U.S. warship in the Black Sea over the weekend. The White House also confirmed on Monday that CIA Director John Brennan was recently in Kiev as part of a European trip.
But the Obama administration is facing familiar pressure to do more. Sen. John McCain, R-Ariz., said he wants the U.S. to provide more arms to the Ukrainians and impose additional and "severe" sanctions on the Russian economy.
"We ought to at least, for God's sake, give them some light weapons with which to defend themselves," he told CBS' "Face the Nation.
McCain also suggested the administration is withholding intelligence from the Ukrainian government and put much of the blame on President Obama and Secretary of State John Kerry.
"Where is the President of the United States?" McCain asked. "Shouldn't the President of the United States be speaking forcefully and strongly? And didn't the president say if they carried out further actions, there would be further sanctions? So far, we haven't heard anything."
In the latest development, a pro-Russian mob on Monday seized a police building in yet another city in Russian-leaning eastern Ukraine, defying government warnings that it was preparing to act against the insurgents.
Dozens of angry men hurled rocks, smashed the windows and broke into a police station in the city of Horlivka not far from the border with Russia, while hundreds of onlookers cheered them on.
Pro-Russian gunmen have seized or blocked government buildings in at least nine cities demanding more autonomy from the central government and closer ties with Russia.
Kiev authorities and Western officials have accused Moscow of instigating the protests, saying the events echoed those in Crimea, which was annexed by Russia last month. Ever since pro-Russian President Viktor Yanukovych fled to Russia in late February, Russia has demanded constitutional reforms that would turn Ukraine into a loose federal state.
After refusing demands for a referendum by separatists in the east, acting President Oleksandr Turchynov indicated Monday that holding a nationwide referendum on the nation's status was a possibility and that such a vote could be conducted on May 25, along with presidential elections. Turchynov expressed confidence that Ukrainians would vote against turning the country into a federation and against its break-up.
Meanwhile, a deadline set by the Ukrainian government for pro-Russian gunmen to leave government buildings in eastern Ukraine and surrender weapons passed early Monday, with no immediate sign of any action to force the insurgents out.
Turchynov had issued a decree Sunday that those protesters who disarm and vacate government offices in several cities in the Russian-leaning east of the country by 0600 GMT Monday will not be prosecuted. Turchynov vowed that a "large-scale anti-terrorist operation" would take place to re-establish control over those areas and that the fate of the Crimean Peninsula, annexed by Russia last month, will not be repeated.
There was no immediate comment from the government on the deadline passing.
But Serhiy Taruta, governor of the Donetsk region, where government buildings in several cities, including the regional capital Donetsk, have been seized by pro-Russian gunmen, said an "anti-terrorist operation" was under way in the region, according to the Interfax news agency.
Taruta did not give any details of what the operation would entail. The governor usually does not have authority to launch such measures on his own and he was likely acting on the orders of top security officials in Kiev.
Late Sunday, the U.S. State Department issued a statement titled, "Russian Fiction: The Sequel, 10 More False Claims about Ukraine."
The document was intended to be a followup to Obama saying last month: "No amount of propaganda can make right something that the world knows is wrong."
The document in part states Russia falsely claims its agents are not in the Ukraine and that the pro-Russia demonstrations are composed exclusively of Ukrainian citizens acting of their own volition.
The Associated Press contributed to this report.