Russian Foreign Minister Sergei Lavrov has warned US Secretary of State John Kerry that imposing sanctions in response to the Ukraine crisis will have a "boomerang effect against the US itself".

In a telephone conversation between the two men, Lavrov said that "hasty and unthought-through steps are capable of causing harm to Russian-US relations".

The US State Department said that Kerry had "underscored the importance of finding a constructive way to resolve the situation diplomatically, which would address the interests of the people of Ukraine, Russia and the international community.

"Secretary Kerry and Foreign Minister Lavrov agreed to continue to consult in the days ahead on the way forward," said the US statement.

On Thursday, the US imposed visa bans on an unspecified number of Russian and Ukrainian individuals. Further visa bans, asset seizures and bans on Russian companies doing business in the US could follow. The EU has suspended talks designed to smooth economic relations with Russia.

Russian gas giant Gazprom said that Ukraine was in arrears to the sum of $1.89 billion, and announced it may cut supplies to the country, as it did in 2009, which in turn disrupted supplies to the rest of Europe. Many of the key gas pipelines to Europe run through Ukraine.

Russian president Vladimir Putin said that following an hour-long conversation with US president Barack Obama their positions remained far apart, continuing the deadlock over the occupation of swathes of east Ukraine by armed Russian units.

The Kremlin claims the units are a local militia, set up by the Russian-speaking eastern Ukrainians to protect themselves following the overthrow of president Viktor Yanukovich in Kiev. This view is widely disputed, with many of their vehicles bearing Russian military markings.

On Thursday, the Crimean parliament, which is dominated by ethnic Russians, voted to join Russia, with a referendum set for 16 March.

Putin said that the Russian military is prepared to support the eastern Ukrainians.

"Russia cannot ignore calls for help and it acts accordingly, in full compliance with international law," he said.

An aide to the Ukrainian border guards' commander, Serhiy Astakhov, said that 30,000 Russian troops were now in the Crimea. The Pentagon believes that 20,000 Russian troops are now in the region.