Ukraine's acting government issued an arrest warrant Monday for President Viktor Yanukovych, accusing him of mass crimes against the protesters who stood up for months against his rule. Russia sharply questioned its authority, calling it an "armed mutiny."

Yanukovych has reportedly fled to the Black Sea peninsula of Crimea, a pro-Russian area in Ukraine. His exact whereabouts are unknown.

Calls are mounting in Ukraine to put Yanukovych on trial, after a tumultuous presidency in which he amassed powers, enriched his allies and family and cracked down on protesters. Anger boiled over last week after government snipers killed scores of protesters in the bloodiest violence in Ukraine's post-Soviet history.

Yanukovych‘s departure came hours after signing an agreement on resolving Ukraine's political crisis that reduced his powers and was seen by many as a tacit admission of defeat. He apparently fled Ukraine's capital by car and aircraft, heading for the parts of the country where he is most likely to find friends, according to the acting head of the police.

The turmoil has turned this strategically located country of 46 million inside out over the past few days. The parliament speaker is now nominally in charge of a country whose ailing economy is on the brink of default and whose loyalties are sharply torn between Europe and longtime ruler Russia.

Russia and the European Union appeared to be taking opposing sides in Ukraine's new political landscape.

Russian Prime Minister Dmitry Medvedev questioned the legitimacy of the new Ukrainian authorities on Monday. According to Russian news agencies, he said the acting authorities have come to power as a result of an "armed mutiny," so their legitimacy is causing "big doubts."

In Brussels, European Commission spokesman Olivier Bailly referred to parliament speaker Oleksandr Turchinov as the "interim president" and said Turchinov will meet with Monday visiting EU foreign policy chief Catherine Ashton in Kiev.

Turchinov said he hopes to form a new coalition government by Tuesday.

White House spokesman Jay Carney told reporters Monday that the U.S. is prepared to provide financial support to Ukraine at this crucial time.

``The United States, working with partners around the world, stands ready to provide support for Ukraine as it takes the reforms it needs to, to get back to economic stability,'' Carney said.

``This support can complement an IMF program by helping to make reforms easier and by putting Ukraine in a position to invest more in health and education to help develop Ukraine's human capital and strengthen its social safety net,'' Carney added.

Russian President Vladimir Putin is facing a huge challenge in how to respond to the turmoil in Ukraine, a country he has declared vital for Russia's interests, which is home to millions of Russian-speakers and hosts a major Russian navy base.

Some in Ukraine's Russian-speaking east and south already have begged the Kremlin to help protect them against what they fear could be violence by the victorious protesters who toppled Ukraine's Moscow-backed leader. Putin has refrained from taking a public stance on Ukraine amid the Sochi Games, but the mounting tensions could quickly leave him with a stark choice: Stick to diplomacy and risk losing face at home, or open a Pandora's box by entering the fray.

If Moscow openly backs separatist-minded groups in Ukraine's Crimean Peninsula that serves as the base for Russia's Black Sea Fleet, it could unleash devastating hostilities that Europe hasn't seen since the Balkan wars. And ignoring pleas for help from pro-Russian groups in Ukraine could shatter Putin's carefully manicured image of the tough ruler eager to stand up to the West, eroding his conservative support base at home, where his foes could be encouraged by the Ukrainian example.

Facing such high risks, Putin has remained silent, weighing his options.

Putin spoke with German Chancellor Angela Merkel by telephone Sunday and the German government said the two agreed that Ukraine's "territorial integrity must be respected."

On Monday, German government spokesman Steffan Seibert told reporters that Ukraine's new leaders should consider the interests of the south and east -- the pro-Russian sections of Ukraine -- in the composition of a new government. He also said the offer of an association agreement with the EU is still on the table.

Meanwhile, Ukraine's acting interior minister, Arsen Avakhov, said on his official Facebook page that a warrant has been issued for the arrest of Yanukovych and several other officials for the "mass killing of civilians."

At least 82 people, primarily protesters, were killed in clashes in Kiev last week.

Yanukovych set off a wave of protests by shelving an agreement with the European Union in November and turning instead for a $15 billion bailout loan from Russia. Within weeks, the protests expanded to include outrage over corruption and human rights abuses, leading to calls for Yanukovych's resignation.

After signing an agreement Friday with the opposition to form a unity government, Yanukovych fled Kiev for his pro-Russian power base in eastern Ukraine. Avakhov said he tried to fly out of Donetsk but was stopped then went to Crimea on Sunday.

Yanukovych then freed his official security detail and drove off to an unknown location, turning off all forms of communication, Avakhov said.

"Yanukovych has disappeared," he said.

Security has been tightened across Ukraine's borders, the Interfax news agency quoted the State Border Guard service as saying.

Avakhov published a letter that he said was from Yanukovych, dated Monday, in which he gave up his security guard. Yanukovych's aides and spokespeople could not be reached Monday to verify the reported letter -- they have been rapidly distancing themselves from him as his hold on power disintegrates.

Activist Valeri Kazachenko said Yanukovych must be arrested and brought to Kiev's main square for trial.

"He must answer for all the crimes he has committed against Ukraine and its people," Kazachenko said.

Tensions have been mounting in Crimea in southern Ukraine. Russia maintains a large naval base in Sevastopol that has strained relations between the countries for two decades.

Pro-Russian protesters gathered in front of city hall in the port of Sevastopol on Monday chanting "Russia! Russia!"

"Extremists have seized power in Kiev and we must defend Crimea. Russia must help us with that," said Anataly Mareta, head of a Cossack militia in Sevastopol.

The head of the city administration in Sevastopol quit Monday amid the turmoil, and protesters replaced a Ukrainian flag near the city hall building with a Russian flag.

As president, Yanukovych moved quickly to consolidate power and wealth, curb free speech and oversee the imprisonment of his top political rival, former Prime Minister Yulia Tymoshenko. But as protesters took control of the capital over the weekend, many allies turned against him.

Tymoshenko, the blond-braided heroine of the 2004 Orange Revolution, is back on the political scene after having been freed from prison.

The current protest movement in Ukraine has been in large part a fight for the country's economic future -- for better jobs and prosperity.

Ukraine has a large potential consumer market, an educated workforce, a significant industrial base and good natural resources, in particular rich farmland. Yet its economy is in tatters.

Ukraine has struggled with corruption, bad government and short-sighted reliance on cheap gas from Russia. Political unrest has pushed up the deficit, sent the currency skidding and may have pushed the economy back into a recession.

The Associated Press contributed to this report.