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.
Britain expressed fears that Russia wanted to disrupt the run-up to presidential elections next month in Ukraine, which has been ruled by an interim government since the overthrow of Moscow-backed president Viktor Yanukovich in February.
Ukraine, which was controlled by Moscow until the Soviet Union collapsed more than two decades ago, has been in turmoil since late last year when Yanukovich rejected closer relations with the European Union and tilted the country back towards Russia. That provoked mass protests in which more than 100 people were killed by police and which drove Yanukovich from office, leading to Kiev's loss of control in Crimea.
In Kiev, Interior Minister Arsen Avakov partly pinned responsibility for the Kharkiv occupation on Russian President Vladimir Putin. "All this was inspired and financed by the Putin-Yanukovich group," he said.
An aide said police went in when the protesters failed to give themselves up and surrender their arms. Officers did not open fire, despite shooting and the grenade attack from the other side, he said. One police officer was badly wounded and some others less seriously hurt.
NO TURNING BACK
In Luhansk, a city of around 450,000 people, protesters have blocked streets leading to the state security building with barbed wire, tires, crates, metal police barriers and sandbags.
Andrei, who said he had stormed the building on Sunday but would not give his family name, said the protesters had 200-300 Kalashnikovs and some stun grenades, but there had been no shooting so far.
"Once you've taken up arms, there's no turning back. We will stay until the authorities agree to hold a referendum on the status of Luhansk," he said.
A standoff also continued in the mining center of Donetsk, Yanukovich's home base, where a group of pro-Russian deputies inside the main regional authority building on Monday declared a separatist republic.
Unlike in Kharkiv, there was no clear sign that further police operations were imminent in the other two cities. "We hope the buildings occupied in Donetsk and Luhansk will soon be freed," acting president Oleksander Turchinov said.
Russia has warned Kiev against using force to end the occupations but authorities may anyway have decided not to give Moscow an excuse to intervene, holding back in the hope that the protests will fizzle out.
In London, Foreign Secretary William Hague said the occupations bore "all the hallmarks of a Russian strategy to destabilize Ukraine".
The West has expressed concern about what it says has been a build-up of Russian forces along the border with Ukraine. Moscow has said the troops are merely taking part in exercises but NATO Secretary-General Anders Fogh Rasmussen urged caution.
"If Russia were to intervene further in Ukraine it would be a historic mistake," he told a news conference in Paris. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."
Lavrov denied responsibility for the trouble in Russian-speaking areas of Ukraine. "One should not seek to put the blame on someone else," he told a news conference in Moscow.
Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian, although they speak Russian as a first language.
Putin will meet his senior officials on Wednesday to discuss economic ties with Ukraine, including on energy, his spokesman said. He gave no details but the Crimea dispute has raised fears that Russia might cut off gas supplies to Ukraine's crippled economy, having nearly doubled the price it charges Kiev.
Kiev missed a midnight deadline to reduce its $2.2 billion gas debt to Russia, although producer Gazprom did not say whether it would take any action against Kiev.
In Brussels, Ukraine's energy minister, EU officials and industry representatives discussed how to reduce reliance on Russian gas.
So far the United States and EU have imposed only mild economic sanctions over the Crimean annexation but some investors are pulling money out of the country. Companies and banks took a net $50.6 billion out of Russia in the first three months of this year, the central bank reported.