Ukraine's government struggled to stay in control of the country's eastern regions as tensions flared Tuesday in three cities. While the government managed to recapture its regional headquarters and detain dozens of pro-Russian protesters in one city, it said "radicals" were keeping 60 people hostage and threatening them in another city.

Unknown "separatists" with weapons and explosives were threatening the hostages inside a security service branch in the city of Luhansk, the Ukrainian Security Service said in a statement Tuesday.

It was not clear who the hostages were or if they were security service employees. The building was seized Sunday by armed pro-Russian protesters.

I call this a theatre of the absurd. It is just artists performing, but the main thing is that there is an ever-dwindling audience - Serhiy Taruta, governor of Donetsk

Earlier Tuesday, Ukrainian authorities battled with pro-Russian protesters but regained control over a government building in Kharkiv, the country's second-largest city, evicting the protesters and detaining dozens.

In Donetsk, a city 250 kilometres further south, protesters dug in for their third day at the 11-story regional administration headquarters they captured on Sunday and began to declare their own parallel government.

Serhiy Taruta, the governor of Donetsk, scoffed at the shifting events in his city.

"I call this a theatre of the absurd," he said. "It is just artists performing, but the main thing is that there is an ever-dwindling audience."

All three cities are in Ukraine's east, where hostility is strong toward the government that took power in February after the ouster of Kremlin-friendly President Viktor Yanukovych. Even though Ukraine's interim authorities have achieved some success in quelling unrest that swept across eastern provinces Sunday, festering discontent threatens to undermine plans to hold a presidential election on May 25.

In Washington, Secretary of State John Kerry on Tuesday threatened Russia with tougher economic sanctions if it fails to back down from its involvement in Ukraine.

"What we see from Russia is an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary," Kerry told the Senate Foreign Relations Committee .

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Kerry called the demonstrations in eastern Ukraine as a "contrived pretext for military intervention just as we saw in Crimea."

Addressing parliament in Kiev, acting President Oleksandr Turchynov said security forces retook control of the Kharkiv administration building early Tuesday but several police were injured in the clashes with separatists.

Interior Minister Arsen Avakov described the measure on his Facebook page as an "anti-terrorist operation."

In a session briefly interrupted by a brawl, parliament also voted to toughen the punishment for undermining Ukraine's national security, imposing jail terms of up to 5 years for separatism.

Masked men take over building

In Donetsk, there was little sign Tuesday evening that government forces had any immediate plan to retake the regional administration building. The city has seen weekly rallies marching on local government offices, but on Sunday groups of masked men carrying batons burst through police lines to take over the building.

A masked man stands atop of a barricade at the regional administration building in in Donetsk. Activists on Monday called for a referendum on the sovereignty of the Donetsk region, which borders Russia, to be held no later than May 11. (Alexander Ermochenko/Associated Press)

By Tuesday, lines of car tires wrapped in razor wire had been erected to deter any possible attempts by police to storm the premises. The tactic appears to have been copied from the anti-government protests in the capital, Kiev, which led to Yanukovych's overthrow. Just like in Kiev, food stations have been created inside the Donetsk building, supplied by volunteers and residents.

No clear leader or agenda has emerged from the obscure group of pro-Russian Donetsk activists behind the standoff.

A declaration adopted Monday claimed sovereignty for what they called the "Donetsk Republic" and demanded a referendum to be held no later than May 11. While none of them have said they necessarily want the region to join Russia, they have also declined to rule out the option.

Despite claims by the separatists groups to represent all of Donetsk, a region of more than 4 million people, rallies outside the regional building since the weekend have drawn crowds only in the low thousands.

Details of 4-way talks not yet firmed

The seizures of the buildings and calls for local votes on secession were an echo of the events that led to Russia's annexation of Ukraine's Crimean peninsula last month. After Yanukovych fled to Russia, Russian troops took control over Crimea and the region voted to join Russia in a hastily called referendum.

The West has not recognized the vote or the annexation and has retaliated with sanctions against Russia.

Even as the United States warned Russia of more sanctions if Moscow makes further efforts to destabilize Ukraine, the White House announced a high-level meeting among U.S., EU, Ukrainian and Russian diplomats in the coming days to try to solve the crisis.

In Moscow, Russia's Foreign Minister Sergey Lavrov said Tuesday that the date and format of the four-way talks haven't yet been agreed. He suggested that Ukraine's presidential candidates could be invited to join the negotiations.

The Kremlin has pushed for a constitutional reform in Ukraine that would turn the country into a federation with broad powers for its regions and ensure its neutrality. The demands reflected Russia's desire to maintain influence over its neighbour and prevent Ukraine from joining NATO.

The new Ukrainian government says Russia has no business telling it what type of government to establish.