Anxious Western powers voiced fear over the growing Crimea crisis Thursday, warning Russia not to escalate tensions and telling pro-Moscow separatists they were playing a "dangerous game."

After dozens of pro-Russian gunmen seized government buildings in the volatile peninsula -- raising fears of a full-blown regional conflict -- NATO leader Anders Fogh Rasmussen tweeted his concern and pleaded with Moscow "not to take any action that can escalate tension or create misunderstanding."

Mounting separatist tensions in Ukraine's Russian-speaking Crimea peninsula have reached dangerous levels and could lead to fresh domestic turmoil or even spark an outright invasion by giant neighbor Russia, analysts say.

On Thursday, pro-Moscow gunmen seized control of the parliament and government headquarters in the regional capital Simferopol, sparking warnings from Ukraine's interim leader that any troop movements at Russia's Black Sea fleet -- based in Crimea -- would be seen as "military aggression".

Ukraine's parliament on Thursday approved the nomination of pro-EU Arseniy Yatsenyuk as the crisis-hit country's new prime minister until presidential polls are held in May.

Yatsenyuk was one of the most prominent leaders of the three-month anti-government protests that swept Ukraine, culminating in deadly violence last week that precipitated the ouster of president Viktor Yanukovych and the collapse of his entire government.

The European Union's foreign policy supremo said Wednesday she would hold talks with Russia's foreign minister next month in Rome, as Brussels and Moscow seek to defuse tensions over Ukraine.

Catherine Ashton announced she would meet Sergei Lavrov in Rome on March 6. "Issues on international agenda to be discussed," she added in a statement posted on Twitter.

U.S. Secretary of State John Kerry on Wednesday renewed allegations against Russia of helping the Syrian regime, accusing Moscow of increasing its aid to President Bashar Assad.

What Assad "is doing is outrageous, unconscionable, unacceptable, disgraceful, craven, it's horrendous. And we all know that. Everybody knows that," Kerry said.

Russia's powerful Investigative Committee on Wednesday called for opposition leader Alexei Navalny to be placed under house arrest as he is probed for fraud.

Navalny, a charismatic Kremlin critic who came second in last year's Moscow mayoral polls and wowed crowds at mass rallies against President Vladimir Putin, is being probed in several cases he claims are politically motivated.

Ukraine's ousted president Viktor Yanukovych drew up a large-scale "anti-terrorist" operation involving 22,000 security forces to "cleanse" protesters from Kiev, leaked documents show -- a plan opponents say would have caused even greater bloodshed.

Parts of the operation -- detailed in official memos handed to a lawmaker by security officials -- appear to have been set in motion when deadly clashes erupted last week between protesters and police. But for unknown reasons, others failed to materialize, allowing protesters to resist and eventually fight back.

Russian President Vladimir Putin on Wednesday ordered a snap check of the battle-readiness of the armed forces in the west and center of the country, including the area bordering Ukraine, as U.S. Secretary of State John Kerry warned that any Russian military moves in Ukraine would be a "grave mistake."

"The commander-in-chief has set the task of checking the capability of the armed forces to deal with crisis situations posing a threat to the military security of the country," said Defense Minister Sergei Shoigu, quoted by the Interfax news agency.

Ukraine's new pro-Western leaders on Wednesday disbanded the country's feared riot police as they sought to win confidence from the splintered and economically ravaged nation in their efforts to forge a unity government.

The interim authorities are grappling the with the dual threats of separatism and a looming debt default as they try to piece the ex-Soviet nation back together following the weekend ouster of pro-Russian president Viktor Yanukovych.

The United States and Britain on Tuesday voiced support for Ukraine amid its political turmoil, vowing it should not be seen as a battleground between East and West.

British Foreign Secretary William Hague after talks with his American counterpart John Kerry also backed Ukraine's territorial integrity, amid fears that the nation with strong links to Russia could be torn apart by the upheaval.