World powers sought to ease tensions over Ukraine Tuesday as the country's interim authorities grappled with the threat of economic collapse and separatism after the dramatic ouster of pro-Moscow president Viktor Yanukovych.

Russia softened its angry tone over the sudden, weekend regime change in Ukraine, while senior EU and US diplomats visited the ex-Soviet country that Yanukovych had tried to steer towards Moscow but whose new leaders have tilted firmly towards the West.

Frantic talks took place between US, European and Russian diplomats as Ukraine appealed for $35 billion (25 billion euros) in aid to avoid bankruptcy and interim president Oleksandr Turchynov warned of a secessionist threat.

The European Union had said it stood ready to give conditional financial assistance to Ukraine.

"In several regions of Ukraine there are very dangerous signs of separatism," Turchynov told parliament on Tuesday, voicing fears that the pro-Russia east could push for partition after a pro-Western administration took charge of the country following months of anti-Yanukovych protests.

Russia had initially reacted with fury to the weekend's rapid-fire political changes -- brought about by last week's clashes that left nearly 100 dead -- accusing the new leadership of waging an "armed mutiny".

But on Tuesday Russian Foreign Minister Sergei Lavrov sought to soften the tone, saying Ukraine should not be forced to choose between Russia and the West.

"We confirmed our principled position of non-intervention in Ukraine's internal affairs," Lavrov said in Moscow.

"We are interested in Ukraine being part of the European family, in all senses of the word," he said. "It is dangerous and counterproductive to force Ukraine into a choice."

EU foreign policy chief Catherine Ashton also sought to calm tensions during a press conference in Kiev.

"We offer support, not interference for the future," Ashton told reporters in a bid to downplay claims that the West wants to bring Ukraine into its sphere of influence.

She also stressed "the importance of the strong links between Ukraine and Russia and the importance of having them maintained".

Ashton however offered no concrete commitments of economic assistance, saying only that the IMF was "very keen" to meet the future new government.

Klitschko stands for president

The tumultuous events of the past week have capped more than three months of relentless protests against Yanukovych's rule sparked by his November decision to spurn a historic pact with the European Union in favour of closer ties with Russia.

One of the stars to emerge during this unrest was Vitali Klitschko, a heavyweight world champion boxer who became one of the three major protest leaders.

The tall 42-year-old has managed to use his sporting credentials to bridge traditional divides in Ukraine between the more nationalist West and pro-Russia East and South, and as such enjoys wide popularity.

Klitschko announced Tuesday he would stand for president in polls set for May 25, shortly after the electoral commission officially kicked off the campaign for elections.

And while Ukraine's opposition-dominated parliament on Tuesday delayed the highly anticipated formation of a new government until Thursday, it voted to apply to the International Criminal Court to prosecute Yanukovych over the "mass murder" of protesters.

Yanukovych, however, has disappeared since he reportedly attempted to flee the country on Saturday from the eastern city of Donetsk.

Fears of separatism

The new government will face the tough task of maintaining financial and territorial stability in Ukraine ahead of the May polls.

On Sunday, thousands of people rallied in the southern port city of Sevastopol -- home to Russia's Black Sea fleet for some 200 years and a bastion of pro-Moscow sentiment -- calling for "Mother Russia" to save them.

Several hundreds still protested Tuesday, while two armoured vehicles had been deployed near Russian military installations in the city centre, in what local media said was a bid to respond to potential "terrorist attacks".

Later Tuesday, the United States and Britain voiced support for Ukraine, vowing it should not be seen as a battleground between East and West.

British Foreign Secretary William Hague, after talks with his US counterpart John Kerry, stressed Ukraine's territorial integrity.

"This is not a zero-sum game, (it) is not a West versus East," said Kerry. "This is about the people of Ukraine and Ukrainians making their choice about their future."

Hague said: "This is a country that needs financial assistance from many sources, including from Russia. It's not about pulling them away from Russia. It's about enabling them to make their own choices."

He also held talks with officials from the International Monetary Fund and is due to visit Ukraine soon.

Deputy US Secretary of State William Burns arrived in Kiev on Tuesday and met Klitschko.

Sviatoslav Shevchuk, the top cleric in Ukraine's Greek Catholic Church, also warned about the risks of civil war and separatism during a visit to Rome, calling on the EU to offer "economic aid and above all diplomatic support".

Hungary's foreign ministry meanwhile said it was "concerned" by reports that Ukrainian far-right group Pravy Sektor broke up a council meeting in a majority ethnic Hungarian town in western Ukraine using "aggression".