At least 12 people have been killed and 30 others wounded during intense clashes in eastern Ukraine, where a helicopter was also shot down by pro-Russian separatists.
Ukraine's Interior Ministry said the dead included four paramilitary police, killed in gun battles with pro-Russian separatists who have seized the town of Slovyansk.
The troops were reportedly ambushed after they pushed into the town on Monday morning.
A statement on the Ukrainian ministry's website did not give details on how the officers died, but a separate statement said Ukrainian troops had started an "anti-terrorist operation" against 800 pro-Russia forces.
Al Jazeera's Paul Brennan, reporting from Slovyansk, said the Ukrainian military had carried out a morning operation to regain control after destroying pro-Russian checkpoints that were surrounding the town.
"The Ukrainian military destroyed roadblocks set up by pro-Russia groups so they could push into the city and engage in gun battles with armed pro-Russia elements," he said.
Arsen Avakov, the interior minister, said there were injuries on both sides.
A pro-Russia militia spokesman in Slovyansk told the Associated Press an unspecified number of people had been killed and wounded in the clashes, including a 20-year-old woman killed by a stray bullet.
Ukraine's Defence Ministry said an Mi-24 helicopter gunship was shot down by heavy machine gun fire while on a combat mission near Slovyansk.
The pilots survived, Ukraine's government said.
It was the third helicopter lost since the Ukrainian military launched its offensive on the rebel-held area.
Slovyansk, a city of 125,000 people, has been the centre of an uprising in east Ukraine against the central government in Kiev, which came to power after the country's Russia-leaning president fled following months of street protests.
Meanwhile, the Ukrainian government sent an elite national guard unit on Monday to re-establish control in the southern port city of Odessa.
Dozens of people had died in the city on Friday in clashes between security forces and separatists. Tensions are running high and protesters freed several prisoners after storming a city police station on Sunday.
In the past few weeks, pro-Russian activists have stormed and seized government buildings and police stations in a dozen eastern Ukrainian cities.