Thirty pro-Russia insurgents and four government troops were killed Monday in operations to expunge anti-government forces around the city of Slovyansk, Ukraine’s interior minister said Tuesday. Rebels said 10 people — fighters and civilians — were killed by Ukrainian troops during clashes Monday. They would not elaborate and there was no immediate way to reconcile the figures.

Gunbattles on Monday around the city of 125,000 were the interim government’s most ambitious effort to date to quell weeks of unrest in Ukraine’s mainly Russian-speaking east.

In the southwest, Kiev authorities also attempted to reassert control over the key Black Sea region of Odessa by appointing a new governor there Tuesday.

This nation of 46 million is facing its worst crisis in decades after its Moscow-leaning president, whose base was in the east, fled to Russia in February following months of protests. Ukraine’s eastern regions, where armed insurgents have seized dozens of government buildings and police stations in recent weeks, are now at odds with western and central Ukraine, which seek closer ties with Europe and largely back the government in Kiev.

Interior Minister Arsen Avakov gave the death toll on his Facebook page Tuesday, adding that 20 government troops were also injured during fighting in Slovyansk. He said about 800 pro-Russia forces in and around Slovyansk were using large-caliber weapons and mortars Monday.

By Tuesday morning, Ukrainian forces had taken hold of a key checkpoint north of the city, dealing a blow to insurgent lines of communication.

In Donetsk, a major city 120 kilometers (75 miles) south of Slovyansk, the airport was closed during the day to international flights following a government order but reopened later.

In the afternoon, about 30 pro-Russia militants armed with automatic rifles and grenade launchers surrounded an Interior Ministry base in Donetsk, demanding that the troops inside not join any government operations against pro-Russia forces. While it was unclear whether they would attack, besieging a government forces base marked an uptick in the offensive of the militants, who previously had focused on seizing police stations and government buildings.

In the southwest, Kiev authorities announced Tuesday they were firing the acting governor in Odessa and replacing him with member of parliament, Ihor Palytsya. Odessa’s police chief was also fired over the weekend.

The move over the predominantly Russian-speaking region came after 46 people died Friday, many in a building fire, after a pro-Ukraine march in Odessa turned into a melee of fighting.

The concern that Odessa could be the next region to fall to pro-Russia forces — particularly after 67 people detained in Friday’s rioting were released by police Sunday under pressure from an angry crowd — has sparked concern in Kiev.

Opposing sides of the Ukraine conflict have traded bitter recriminations over the Odessa deaths. As residents gathered on Tuesday to lay flowers near the building, they remained confused about exactly what caused the fire and suspicious of the police forces who for whatever reason did not stop the bloodshed.

Those who gathered to commemorate the victims, at least 15 of whom were buried Tuesday, fell in both the pro-Ukraine and pro-Russian camps. But most people placed responsibility for what happened on the police for standing aside during the violence and for later releasing the pro-Russia activists.

“I saw how the police were on the side of the pro-Russians — they broke rank when the Russians attacked and closed ranks when they stepped back,” said 46-year-old Vitaly Khadyko.

The central government attempted to boost confidence by sending in an elite national guard unit, which could be seen patrolling the streets of Odessa. On Monday, newly appointed police chief Ivan Katerinchuk left his office to talk to activists who had gathered outside.

“We won’t allow such scenarios to be repeated,” he told the angry crowd.

The goals of the pro-Russian insurgency are ostensibly broader powers of autonomy for the region, but some insurgents do favor separatism or even joining Russia.

Leaders of the anti-government movement say they plan to hold a referendum on autonomy for eastern regions on Sunday, although no visible preparations for the vote have yet been seen.

Russia has put the blame for the unrest squarely on the interim government in Kiev. During a Tuesday meeting in Vienna with the Council of Europe, Russian Foreign Minister Sergey Lavrov demanded that the Ukrainian government end its armed assaults on rebel strongholds. He said he was open to another round of international talks to ease the crisis, but only if pro-Russia rebels were included.

“Those who protest … want their voices heard,” he told reporters. “They want to have an equal voice when it comes to deciding the fate of their own country.”

His Ukrainian counterpart rejected the proposal, saying the Kiev government already represents all the people of Ukraine.

Ukrainian authorities have blamed Moscow for fomenting the unrest in the east, saying it’s an attempt to derail Ukraine’s May 25 presidential election. Lavrov repeated Moscow’s claims that violence in Ukraine proved the country was unready for a vote, and that a constitution allowing for greater federalization should come before a presidential election.

“Scheduling elections in times when the army is used against parts of the population is not conventional,” he said. “This is not Afghanistan.”

Russia, Ukraine and European and U.S. leaders met in Geneva on April 17 and signed a deal calling for the dissolution of all illegal military formations in Ukraine. But the sides quickly accused each other of violating the agreement, which has done little to mitigate the turmoil in the country’s east.

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By Peter Leonard and Yuras Karmanau. Yuras Karmanau reported from Odessa, Ukraine. George Jahn contributed reporting from Vienna, Austria.