John Bacon, and Maureen Groppe

USA TODAY

Ukrainian troops are battling a pro-Russia militia occupying an eastern city in an apparent escalation of a security sweep to bring the region back under government control.

Gunfire and multiple explosions could be heard Monday in Slovyansk, a city of 125,000 people that has become the focus of the armed anti-government insurgency.

The Interfax news agency cited Interior Minister Arsen Avakov as saying that pro-Russia forces were deploying large-caliber weapons and mortars.

In the beleagured city of Odessa, pro-Russian demonstrators stormed police headquarters on Sunday and won the release of 67 people detained after deadly riots there.

It was Odessa's latest clash between pro-Russian separatists and supporters of the Ukrainian government in the port city, where more than 40 people died in the riots and subsequent fire in a trade union building on Friday.

Earlier Sunday, Ukraine Prime Minister Arseniy Yatsenyuk visited Odessa and accused Russia of engineering violence that left dozens of people dead in recent days.

Yatsenyuk also blamed government security forces for failing to prevent the bloodshed, calling the deaths a "tragedy of all of Ukraine."

But he offered no words of compromise to Russia, accusing it of a "well-prepared and organized action against people, against Ukraine, and against Odessa."

Odessa Police Chief Petr Lutsyuk was fired Saturday. Yatsenyuk said investigators will determine the cause of the breakdown in police and security forces that allowed the unrest to become so deadly. But he also promised that prosecutors will bring to justice all Russian-backed organizers and instigators.

Yatsenyuk's visit came one day after pro-Russia militants released seven European military observers as the Ukraine government pressed ahead with a military offensive to take back control of cities from the militancy.

Military observers from the Organization for Security and Cooperation in Europe were held for more than a week on what militants said were charges of spying. The OSCE, a diplomatic group, said its members were grabbed as bargaining chips.

U.S. Secretary of State John Kerry said the release was a step in the right direction. But he pressed Russia to stop backing separatists and to help oust them from government buildings seized in about a dozen cities and towns.

Vyacheslav Ponomarev, self-declared "people's mayor' of the town of Sloviansk outside Odessa, says he holds an unspecified number of other captives. The prisoners are believed to include Ukrainian journalists, activists and politicians.

President Obama has threatened to impose more financial sanctions on Moscow if it does not stop assisting the militancy, but Sen. Marco Rubio, R-Fla., said Saturday that Obama has failed to take strong enough action to alter the situation

"President Obama talks tough about Vladimir Putin. But his actions have not gone far enough to change Putin's calculation that the benefits of his aggression outweigh the costs," Rubio said.

Senate Republicans have introduced a bill that would increase sanctions and provide Ukraine with defensive military assistance, something Obama has refused to do.

Interior Minister Arsen Avakov said Ukrainian forces were continuing to gain on rebels and had seized control of a TV tower in Kramatorsk, near the rebel stronghold of Slovyansk.

"We are not stopping," Avakov said Saturday. It was Avakov who fired Lutsyu hours after the chief issued a statement calling for calm in the city of 1 million.

Russian President Vladimir Putin's spokesman said the Odessa deaths were evidence that the government in Kiev, which came to power following the toppling of the pro-Russia president after months of protests, encourages nationalist extremists.

"Their arms are up to their elbows in blood," Russian news agencies quoted Dmitry Peskov as saying.

Putin spoke by telephone Sunday night with German Chancellor Angela Merkel, the latest in a series of discussions they have had about Ukraine. The Kremlin said they agreed on the importance of the role to be played by the Organization for Security and Cooperation in Europe, and said Swiss President Didier Burkhalter, whose country currently chairs the OSCE, would visit Moscow on Wednesday.

Odessa is the major city between the Crimean peninsula, which Russia annexed in March, and the Moldovan separatist region of Trans-Dniester where Russia has a military peacekeeping contingent. Concerns are mounting that Russia ultimately aims to take control of a huge swath of Ukraine from Trans-Dniester to the east.

The fate of those killed in the trade union building has already become a rallying cause for resistance to the authorities by pro-Russians in the east. In a position eagerly promoted by the Kremlin, critics of the government have blamed those deaths on radical ultranationalists abetted by the government.

Pro-unity activists have argued, meanwhile, that their rally came under assault, including from attackers bearing firearms, leading to the deadly blaze.

Contributing: Associated Press