(Reuters) - U.S. Secretary of State John Kerry accused Russian agents and special forces on Tuesday of stirring separatist unrest in eastern Ukraine, saying Moscow could be trying to prepare for military action as it had in Crimea.

Armed pro-Moscow protesters were still occupying Ukrainian government buildings in two cities in the largely Russian-speaking east on Tuesday, although police ended a third occupation in a lightning night-time operation.

Ukraine's security service said separatists occupying the security headquarters in Luhansk had planted bombs in the building and were holding as many as 60 hostages. Activists in the building denied they had explosives or hostages, but said they had seized an armory full of automatic rifles.

The Ukraine government says the occupations that began on Sunday are part of a Russian-led plan to dismember the country. Kerry said he feared Moscow might repeat its Crimean operation.

"It is clear that Russian special forces and agents have been the catalyst behind the chaos of the last 24 hours," he said in Washington, and this "could potentially be a contrived pretext for military intervention just as we saw in Crimea."

Moscow annexed the Black Sea peninsula last month after a referendum staged when Russian troops were already in control.

Earlier, Russian Foreign Minister Sergei Lavrov dismissed Western accusations that Moscow was destabilizing Ukraine, saying the situation could improve only if Kiev took into account the interests of Russian-speaking regions.

Shots were fired, a grenade thrown and 70 people detained as Ukrainian officers ended the occupation in the city of Kharkiv during an 18-minute "anti-terrorism" action, the interior ministry said.

But elsewhere in Ukraine's eastern industrial heartland, activists armed with Kalashnikov rifles and protected by barbed-wire barricades vowed there was no going back on their demand -- a vote on returning to Moscow rule.

In the city of Luhansk, a man dressed in camouflage told a crowd outside the occupied state security building: "We want a referendum on the status of Luhansk and we want Russian returned as an official language."

The Kremlin's standoff with the West has knocked investors' confidence in the Russian economy, and the International Monetary Fund on Tuesday cut its forecast of growth this year to 1.3 percent, less than half the 3 percent it had originally projected.