Russia on Friday declared it reserved the right to protect compatriots in the whole of Ukraine, seen as a threat that Moscow could move its forces beyond the Russian-speaking peninsula of Crimea.
The Kremlin's latest statement comes after a protester was killed in clashes in the east
of the ex-Soviet country on Thursday, and as Russian foreign minister Sergei Lavrov and US secretary of state John Kerry hold talks on the Ukraine crisis in London.
Russian forces have occupied Ukraine's Russian-speaking Crimean peninsula and thousands of troops have staged drills near the country's eastern borders.
"Russia recognises its responsibility for the life of its compatriots and fellow citizens in Ukraine and reserves the right to protect people," the Russian foreign ministry said.
The pro-Moscow parliament in Crimea has called for a referendum on Sunday to decide whether the flashpoint peninsula should join the Russian Federation. On Friday Crimea's leader said east Ukraine should do the same.
In the east Ukrainian city of Donetsk on Thursday, a pro-Kiev protester was stabbed and killed and several were injured in clashes, marking the first death since tensions erupted in the southeast of the country after pro-Russian forces took control of Crimea.
"We have repeatedly stated that those who have come to power in Kiev should disarm militants, ensure the safety of the population and people's legal right to hold rallies," the Russian foreign ministry said.
"Unfortunately, as the events in Ukraine demonstrate, this is not happening, (and) the Kiev authorities do not have the situation under control."
Historic and linguistic faultlines divide the ex-Soviet nation of 46 million into the Russian-speaking, Moscow-leaning east and the Ukrainian-speaking, pro-European Union west.
Russia stunned the West by sending troops to the Crimean peninsula after a popular uprising in Kiev ousted pro-Moscow president Viktor Yanukovych last month.
Observers have warned that the raging crisis could ultimately split Ukraine.
On Sunday, Crimea is preparing to hold a referendum which is expected to cement Russia's takeover of the strategic peninsula, home to its Black Sea fleet.
Putin acknowledged last week Russia has the capability to occupy the whole of Ukraine but said at the time it was not necessary.
Political analyst and Kremlin adviser Dmitry Orlov said he was not expecting Russia to launch a full-scale assault against Ukraine.
"This is a signal to aggressive groups in Ukraine who are seeking to destabilise the situation in the country," he said, referring to the statement.
Independent analysts said it was difficult to predict whether the Kremlin was bluffing or was getting ready to send its troops into eastern Ukraine.
"Will Russia barge into Ukraine's east? This cannot be explained rationally," said Alexander Konovalov, president of the Institute of Strategic Assessments.
Konovalov did not rule out that the threat was timed to coincide with the crisis talks between Lavrov and Kerry.
"If they don't listen to us we'll go even further," Konovalov said, referring to the Kremlin's apparent line of thinking.
The West has slammed the referendum set for Sunday as illegal and threatened to slap sanctions against top Russian officials unless Moscow withdraws from Ukraine.
Russia counters that the plebiscite is legal and that all peoples have a right to self-determination.
Maria Lipman, an analyst with the Carnegie Moscow Center, said Russia's rhetoric was becoming increasingly confrontational in the face of Western threats of sanctions.
"Things have gone so far that neither side is willing to retreat," she said.
But Nikolai Petrov, a professor at the Higher School of Economics, suggested Russia would be willing to bargain over the future of Ukraine.
"Russia will take away Crimea in any case," said Petrov.
"As far as eastern Ukraine is concerned, it proposes to negotiate on just how Russia will control it."
"Russia's main plan is: let western Ukraine join Europe while eastern Ukraine will remain within the Russian sphere of influence."
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