Hundreds of pro-Russians remained holed up Tuesday inside the Donetsk administration building in eastern Ukraine a day after proclaiming the creation of a sovereign "people's republic" and demanding that an independence referendum be held before May 11, as the U.S. said Russia's takeover of Crimea could prompt it to review its military presence in Europe.

"We have formed a provisional government in Donetsk," separatist leader Vadym Chernyakov told Agence France Presse inside the occupied building.

U.S. Secretary of State John Kerry warned Tuesday that Russia was sending agents into eastern Ukraine to "create chaos" the Kremlin could use as a pretext for more military intervention.

With tensions again on the boil in Ukraine, Kerry said he would meet next week with Russian Foreign Minister Sergei Lavrov as Washington seeks to tamp down the most serious East-West crisis since the Cold War.

NATO's chief on Tuesday warned Russia against intervening further in Ukraine, urging Moscow to "step back" after pro-Kremlin militants seized government buildings in several cities in the east.

"Russia's illegal aggression against Ukraine is the greatest challenge to Europe's security in a generation," Anders Fogh Rasmussen told reporters.

The leader of Moldova's pro-Moscow breakaway region on Monday urged the country's politicians to start discussions that would allow Transdniestr to formally join Russia.

"Our dream is a prosperous independent Trasndniestr together with Russia," Yevgeny Shevchuk said in a state of the nation address.

Germany said Monday it was worried by the actions of pro-Russian demonstrators in eastern Ukraine and called for stability in the region.

German government spokesman Steffen Seibert said Berlin was watching events in the eastern Ukrainian cities Donetsk and Kharkiv closely after irate activists occupied government buildings there Sunday.

The United States called Monday on Russia's President Vladimir Putin to stop “destabilizing” Ukraine, saying it was concerned about "several escalatory" moves over the weekend, as Moscow called for the federalization of the unrest-hit country.

White House spokesman Jay Carney said the United States was prepared to impose further sanctions, this time targeting the Russian economy, if the situation continues to escalate.

Russian President Vladimir Putin has ordered a crackdown on "destructive" non-government organizations, saying such groups were behind the mass protests that brought down Ukraine's government.

Moscow has repeatedly accused the new authorities in Kiev and the protesters who toppled pro-Moscow President Viktor Yanukovych of being extremists, nationalists and even "Nazis".

Bank of Cyprus said Monday it is looking "positively" at a revised offer to sell its loss-making PJSC Ukrainian unit to Russia's Alfa Group as part of a restructuring drive.

In January, BoC, the island's largest lender, said it had agreed to sell its 99.77 percent stake in the subsidiary to ABH Ukraine, which is part of Alfa Group, for 225 million euros ($308 million).

Ukraine faced a fresh secessionist crisis Monday when pro-Russians occupying the Donetsk government seat proclaimed independence from Kiev and vowed to hold a referendum on joining Kremlin rule.

The declaration and subsequent appeal for Russian military help should Kiev's new pro-Western leaders resist their independence drive put the nation of 46 million in danger of disintegration and added still more pressure on Western powers to act.

U.S. Defense Secretary Chuck Hagel got a first-hand look at China's new aircraft carrier on Monday as Beijing offered an unprecedented glimpse into its growing military clout.

Hagel praised the ship tour as a promising step forward for a fledgling U.S. dialogue with the often secretive People's Liberation Army (PLA), but the two sides remained deeply divided on cyber-spying and regional territorial disputes.