Moscow could choose to openly back separatist-minded groups in Ukraine's Crimean Peninsula that hosts the base for Russia's Black Sea Fleet, further splitting Ukraine. Ignoring pleas for help from pro-Russian groups in Ukraine could shatter Putin's carefully manicured image of the tough ruler eager to confront the West, encouraging his foes back home.

His premier, Dmitry Medvedev, on Monday unleashed scorn on the new Ukrainian authorities who replaced President Viktor Yanukovych and questioned their legitimacy. But he wouldn't say what action Russia might take to protect its interests.

"If you consider Kalashnikov-toting people in black masks who are roaming Kiev to be the government, then it will be hard for us to work with that government," Medvedev said.

The Russian foreign ministry criticised the West for ignoring what Moscow said was the opposition's decision to renege on its agreement to form a unity government. The aim, Moscow said, was to use "dictatorial and, sometimes, even terrorist methods" to "suppress dissent in various regions of Ukraine".

Putin's best hope for striking a peaceful compromise on Russian interests in Ukraine could paradoxically be former prime minister Yuliya Tymoshenko, who was freed on Saturday after more than two years behind bars.

Tymoshenko, who narrowly lost the 2010 presidential vote to Yanukovych and was jailed on charges of abusing her office, immediately jumped into the limelight. She flew to the capital and spoke to tens of thousands of demonstrators on Kiev's Independence Square.

Tymoshenko's charisma, ambitions and unparalleled political skills would make her all but certain to win the presidency in early elections set for May. Putin, who had good ties with Ukraine's fiery ex-premier in the past, could hope to strike a deal with her that would safeguard Russian interests without the need to resort to force.

"If she consolidates power, Putin will be quite happy. They understand each other perfectly well," said Stanislav Belkovsky, a political consultant who advised the Kremlin and worked in Ukraine. "He has good ties with Tymoshenko, and her triumph would suit him."

Tymoshenko, who comes from eastern Ukraine, might be able to restore an uneasy balance between Ukraine's Russian-speaking east and south, and its western regions that abhor Russian influence.

But she is burdened by the legacy of insider deals and corruption allegations during her business and government careers. She also faces the tough task of winning the trust of protesters who are suspicious of old players and want strong action. And she will have to walk a fine line between publicly taking an anti-Kremlin posture to win votes in western regions and assuaging residents of the east that their interests will be protected.

Tymoshenko could make a more convenient partner than the hesitant and indecisive Yanukovych, who had tried to manoeuvre between Russia and the West and provoked many Ukrainians by abruptly shelving a pact with the European Union in favour of a bailout from Moscow.

Russia's state-controlled broadcasters heaped scorn on Yanukovych, casting him as too weak to establish order and who betrayed police as they stood behind him. That's a sign the Kremlin sees him as a discarded asset.

Reports about Yanukovych hiding in the Crimea, which hosts Russia's naval base, could encourage some activists in Kiev and western Ukraine to pressure the government to apprehend him. They want to put him on trial for murder.

Watch: Mixed opinions over Yanukovych's downfall in his Ukraine home town

That could provoke residents of the Crimea, where most of the population speaks Russian and abhors nationalist groups from western Ukraine.

Any clashes would put pressure on Putin to intervene, and he could come under the influence of more hawkish figures in his administration who have been advocating a tough line on Ukraine to expose alleged Western plots to pry the country from Russia's sphere of influence.

Crimea fell under Russia's control in the 18th century under Catherine the Great and only became part of Ukraine in 1954, when then-Soviet leader Nikita Khrushchev transferred it from Russian to Ukrainian administrative control.

Speculation is rife that Crimea could become the target of Kremlin ambitions and host a possible secessionist plot that would rip Ukraine apart. All eyes are on Sevastopol, the historic home of Russia's Black Sea fleet. Russian media said Yanukovych - last spotted in the resort of Balaklava - may have taken refuge on a Russian military boat in the port.

Angry crowds have gathered in Sevastopol. They reject Kiev's new opposition-led government and want to install as mayor Alexei Chalov, a Russian who favours union with Moscow. On Monday, a Russian flag was hoisted above Sevastopol's city hall.

Fyodor Lukyanov, the editor of magazine and head of the Council for Foreign and Defence Policies, said the Kremlin would watch how the new Ukrainian government dealt with the Black Sea Fleet and authorities' pledges to stay away from military blocs.

He said that if Moscow saw Kiev reneging on these issues, it would set off alarms in the Kremlin as a possible signal that Ukraine intended to join Nato.

"Ukraine in Nato has been a red line," Lukyanov said. "If that happens, various options will come under consideration, including appeal to certain parts of Ukraine, including the Crimea."

A violent confrontation between pro- and anti-Moscow forces could force Russia to act.

"If clashes occur in the Crimea, Russia will start by issuing harsh statements and put the Black Sea Fleet on high alert," Lukyanov said. "Russia couldn't ignore it. There are all kinds of risks."

More than half of Crimea's two million inhabitants are ethnically Russian. The Russian fleet employs 25,000. Unlike most of Ukraine, where statues of Lenin were felled in a revolutionary frenzy, Sevastopol's giant Lenin still gazes over the Black Sea.