MOSCOW, April 23 - Russian Foreign Minister Sergei Lavrov accused the United States of being behind the political upheaval in Ukraine and said Moscow would respond if its interests came under attack, Reuters reported.

Lavrov's comments came a day after U.S. 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.

The crisis in Ukraine, now in its fourth month, has dragged Russia's relations with the West to their lowest 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, Lavrov said Moscow would respond if its interests, or the interests of Russian 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.

"There is no reason not to believe that the Americans are running the show," RT quoted him as saying.

Washington said that suggestion was "ludicrous", while NATO's deputy secretary general, Alexander Vershbow, said Russia must de-escalate the situation and avoid "inflammatory rhetoric and misrepresentations of the situation inside Ukraine".

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 called on Moscow to pull troops back from the border, saying it feared pro-Russian separatists could use their proximity to provoke a Russian invasion.

Lavrov's ministry, in a separate statement, 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 that applies not only to the pro-Russian separatists in the east, but also to groups in Kiev whose protests helped bring Ukraine's new government to power.

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.

Ukraine's SBU state security service warned that it would attack militants who held out. It said the Geneva accord required all illegal militias to lay down their arms:

"If not, the law enforcement agencies will use all their forces, means and capabilities to put an end to criminal activities and restore law and order and communal security."

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. (rt/ez)