Ukraine resumed a military offensive against pro-Russian forces in the country's east on Saturday, as military observers from the Organization for Security and Cooperation in Europe who were held for more than a week were released.
Interior Minister Arsen Avakov said Ukrainian forces had seized control of a television tower in Kramatorsk, near the rebel stronghold of Slovyansk where at least three were killed in fighting on Friday, Reuters reported.
"We are not stopping," Avakov wrote on his Facebook page early Saturday.
News reports claimed additional fighting broke out in the city of Kramatorsk, south of Slovyansk, on Saturday.
Tensions were especially high after at least 42 people died in clashes between government supporters and opponents in the Black Sea port of Odessa on Friday. The clash began with street fighting between two sides in which at least three people were reported killed by gunfire, then turned into a grisly conflagration when government opponents took refuge in a building that caught fire after protesters threw firebombs inside.
The city's police chief, Petr Lutsyuk, issued a statement calling for calm in the city of about 1 million, but hours later he was fired by Avakov.
There were also signs of a desire for revenge. A page appeared on Vkontakte, a Russian analogue of Facebook, showing photos and stating home addresses of people allegedly responsible for the fire deaths. In Donetsk, the largest city in the insurgent east, demonstrators who stormed the local office of the Ukrainian Security Service on Saturday shouted, "We will not forgive Odessa."
Peskov, the spokesman for Russian President Vladimir Putin, said the bloodshed demonstrated the acting government's tolerance of or collusion with nationalist extremists and had driven efforts to resolve the crisis into a dead end.
Russia's Foreign Ministry said the fatal fire was "yet another manifestation of the criminal irresponsibility of the Kiev authorities who indulge insolent radical nationalists ... which are engaging in a campaign of physical terror" against those in Ukraine who want more autonomy for the pro-Moscow regions.
Meanwhile, insurgent leader Vyacheslav Ponomarev announced Saturday that all seven military observers and their five Ukrainian assistants had been released.
The observers were seized on April 25 in the city of Slovyansk, the epicenter of eastern Ukraine's unrest, as they traveled with an Organization for Security and Cooperation in Europe observer team. The insurgents said they possessed unspecified suspicious material and alleged they were spying for NATO.
An observer from Sweden was also seized as part of the team, but was released earlier. Unlike the other observers' countries, Sweden is not a member of NATO.
One of the observers, German Col. Axel Schneider, told The Associated Press that all 12 of the detainees held up well.
"They had a very good attitude and that gave them the strength to stand the situation," he said.
Those held included three other Germans and one soldier each from Poland, Denmark and the Czech Republic.
Oleksandr Turchynov, Ukraine's acting president, said many insurgents were killed or wounded Friday in the eastern offensive that also underlined the military's vulnerability. The military action came two days after Kiev said it had lost control of eastern Ukraine.
Both sides said two Ukrainian helicopters were shot down by the insurgents near Slovyansk, killing two crew members, while authorities said another seven people also died: three separatist gunmen, two soldiers and two civilians.
By nightfall, Ukrainian troops and armored personnel carriers blocked all major roads into Slovyansk, and the central part of the city remained in the hands of pro-Russia gunmen, according to Associated Press journalists inside. Most shops were closed, and the few that were open were crowded with customers stocking up on supplies.
Sporadic gunfire was heard in Slovyansk's downtown late Friday, while Russian news reports said there were armed clashes in Kramatorsk. There was no immediate independent confirmation of fighting.
The Ukrainian Security Service said one helicopter was downed with a surface-to-air missile, adding that the sophisticated weapon undercut Russia's claims the city of 125,000 people was simply under the control of armed locals.
"Ukrainian security forces so far are not ready for large-scale military actions; moreover, such actions could provoke Russia's invasion," said Kiev-based political analyst Volodymyr Fesenko.
Russia has massed tens of thousands of troops in areas near Ukraine's border. Kiev claims Moscow is preparing to invade and that it is fomenting the unrest in the east, where insurgents have seized government buildings in about a dozen cities and towns. The Kremlin denies the allegations, but Foreign Minister Sergey Lavrov has warned Russia would respond to attacks on its citizens or interests in the east.
In Washington, German Chancellor Angela Merkel and President Obama warned that Russia could be hit by new sanctions from the U.S. and the European Union if it continues disruptive actions in Ukraine. Previous sanctions are showing signs of significant effect on the Russian economy.
Slovyansk is strategically key because Ukraine has a huge stockpile of automatic rifles and other light weapons near the city, according to a commentary Friday for Britain's Royal United Services Institute defense think-tank.
"Today, Kiev has upped the ante in the standoff and will test Russian resolve to prevent the Ukrainian government from regaining control of the city and its light weapons stockpile," the think-tank authors wrote.
At Russia's request, the U.N. Security Council met in an emergency session Friday on Ukraine.
Russian ambassador Vitaly Churkin demanded a "swift halt of all violence," but Western powers scoffed at his country's indignation.
"Russia ... has released bands of thugs on Ukraine ... and is suddenly discovering this mixture might escape its control," French ambassador Gerard Araud said.
Ukraine, a nation of 46 million, is torn between those in the west who favor closer ties with Europe and many Russian-speakers in the east who look toward Moscow. Separatists who have seized government buildings in at least 10 eastern cities amid fears that Moscow is seeking a pretext to invade or annex more territory like it did with Ukraine's Black Sea peninsula of Crimea in March.
The peace deal in Geneva last month aimed to get those who had seized government buildings in Ukraine to leave and calm down the tensions that have prompted the United States and the European Union to slap Russia with rounds of sanctions.
The Russian Foreign Ministry accused Ukraine's government of using "terrorists" from ultranationalist organizations for Friday's military operation. It also claimed that Kiev deployed tanks and helicopters that were "conducting missile strikes on protesters," something that neither side in Ukraine reported.
Russia also cited insurgents in Ukraine as saying that some of the government attackers spoke English — an insinuation that the Ukrainian military was getting help from the West.
Kiev's interim government came to power after Yanukovych fled, drummed out by months of anti-government protests. Ukraine plans to hold a presidential election on May 25.
The Associated Press contributed to this report.