SLOVYANSK, Ukraine — Pro-Russia insurgents in eastern Ukraine on Saturday released the seven OSCE military observers and five Ukrainian assistants who had been held for more than a week.

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 and the Swede reportedly suffers from a mild form of diabetes.

Shortly before the release, the insurgents’ leader in Slovyansk, Vyacheslav Ponomarev, was quoted by the Interfax news agency as saying he ordered the release because of increasing insecurity in the city.

Ukraine on Saturday expanded a military operation to seize ground from rebels in the east, following the bloodiest day in the country since the Western-backed government in Kiev came to power.

Interior minister Arsen Avakov said his forces had launched a dawn raid near the town of Kramatorsk, taking back a TV tower as Kiev struggles to wrest back control of the east amid a pro-Russian insurgency that threatens to tear the country apart.

Two Ukrainian helicopters were reported shot down outside the city on Friday, killing two crew members and the Ukrainian Defense Ministry said two other soldiers were killed in a clash on the outskirts. Ponomarev said 10 local people were killed in a confrontation with soldiers on Slovyansk’s outskirts, but there was no independent confirmation.

Tensions in Ukraine heightened sharply 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 the two sides in which as 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.

At least 36 people were killed in the fire, according to the emergencies ministry. An Interior Ministry statement gave the overall death toll for the day at 42, but did not give a breakdown.

The city’s police chief, Petr Lutsyuk, on Saturday issued a statement calling for calm in the city of about 1 million, but hours later he was fired by Avakov.

Russian President Vladimir Putin’s spokesman on Saturday decried the Odessa deaths as evidence that the interim government in Kiev encourages nationalist extremists.

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

Although Russia denies allegations that it is fomenting the unrest in eastern Ukraine, where insurgents have seized government buildings in about a dozen cities and towns, it sent human rights ombudsman Vladimir Lukin to negotiate for the release of the observers.

Lukin was quoted by the Russian state news agency RIA Novosti as saying the release was “a voluntary humanitarian act.”