Russian foreign minister Sergei Lavrov has accused the United States of being behind the political upheaval in Ukraine and said Moscow would respond if its interests came under attack.

Mr Lavrov's comments came a day after US vice president Joe Biden was in the Ukrainian capital with promises of support for the pro-Western government, and a warning to Russia not to interfere in Ukraine.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 3 seconds 3 m 3 s Listen to Barbara Miller's report Download 1.4 MB

The crisis in Ukraine, now in its fourth month, has dragged Russia's relations with the West to their lowest point since the Cold War.

In the east, pro-Russian armed separatists have seized about a dozen public buildings and are defying Kiev's authority.

A further escalation could lead to damaging economic sanctions, and raises the risk of a disruption to the Russian gas supplies on which Europe depends.

NATO says Russia has built up a force of about 40,000 troops on its border with Ukraine. Moscow says some are stationed there permanently, while others have been deployed as a precaution to protect Russia from the instability in Ukraine.

In Moscow, Mr Lavrov said Russia would respond if its interests, or the interests of its citizens, were attacked.

"Russian citizens being attacked is an attack against the Russian Federation," he said according to excerpts of an interview with the Russia Today news channel.

In reference to developments in Kiev, he said: "There is no reason not to believe that the Americans are running the show."

The US State Department dismissed Mr Lavrov's comments as "ludicrous".

"Many of the claims he made in the interview are ludicrous and they are not based in fact," State Department spokeswoman Jen Psaki said.

Asked for details, she replied: "One was certainly that the United States has anything to do with Ukraine's counter-terrorism operations... or that we're 'running the show' or funding it. I would put those all in the 'ludicrous' category."

Russia justified its intervention in Crimea earlier this year by saying it had to defend Russians living there. In eastern Ukraine, some people hold Russian passports.

Ukraine steps up security operation against separatists

In a separate statement, Mr Lavrov's ministry accused the United States and the interim government in Kiev of a "distorted interpretation" of an international accord, signed in Geneva last week, under which illegal armed groups in Ukraine are to disarm and give up buildings they have occupied.

Russia said the condition applied not only to the pro-Russian separatists in the east, but also to groups in the Ukrainian capital whose protests helped bring Ukraine's new government to power.

"Instead of taking effective measures to implement the... agreements, Kiev, Washington and a series of European capitals continue to insist that it is only Ukrainian citizens defending their rights in the south-east of Ukraine who need to give up their weapons," a ministry statement said.

Earlier, Ukraine's government re-launched a security operation to crack down on the pro-Russian armed groups after an Easter pause and said it had the backing of the United States.

But it was unclear what steps Kiev could take to restore its authority in the mainly Russian-speaking east, without wrecking the Geneva deal.

"The security forces are working on the liquidation of illegal armed groups," in the east of Ukraine, first deputy prime minister Vitaly Yarema told reporters.

"The corresponding activities will be carried out in the near future, and you will see the results."

The Interior Ministry said it had flushed armed separatists out of a town which they had controlled in eastern Ukraine in an "anti-terrorism" operation.

It said the operation took place on the outskirts of the town of Sviatogorsk and that no-one was injured. There had been no previous reports of gunmen in the town, which lies just outside the stronghold of pro-Russian militants in Slaviansk.

Kiev's decision to resume its security operation in the east was prompted in part by the discovery of two bodies in a river in eastern Ukraine.

One body was that of Volodymyr Rybak, a member of the same party as Ukraine's acting president.

Reuters