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".

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.

The Communist Party won a surprise victory against President Vladimir Putin's ruling party in a mayoral race in Russia's third largest city of Novosibirsk, election officials said on Monday.

Communist lawmaker Anatoly Lokot won 43.75 percent of the vote against the acting mayor Vladimir Znatkov from the ruling United Russia party, who took 39.57 percent, the Siberian city's electoral commission said.

Progressive Socialist Party leader MP Walid Jumblat held talks recently in Paris with senior French officials, who stressed the need to stage the Lebanese Presidential elections within the constitutional time-frame, local newspapers reported on Monday.

According to al-Joumhouria newspaper, French officials briefed Jumblat, who is on a private visit to France, the importance of electing a strong president capable of facing the current challenges.

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.

Jordan's King Abdullah II is to travel to Moscow on Wednesday for talks with Russian President Vladimir Putin on the Syrian conflict, after visits to the Vatican and Austria, the palace said.

The king will stop Monday in the Vatican to meet with Pope Francis, who is to visit Jordan as part of a Holy Land tour in May, before heading to Austria on Tuesday.