Dozens of people were killed in a fire and others were shot dead when fighting between pro- and anti-Russian groups broke out on the streets of Odessa on Ukraine's Black Sea coast on Friday, opening a new front in a conflict that has split the country.
In the east, pro-Russian separatists brought down two Ukrainian military helicopters involved in a pre-dawn operation to try to dislodge the militants from their strongholds in the town of Slaviansk.
The separatists said three of their number had been killed, and two civilians, while the defense ministry said two crew from the downed helicopters died and two other servicemen were killed when separatists attacked them on Friday evening.
"Heavy fighting is continuing," the Ukrainian Defence Ministry said.
Russian-speaking eastern Ukraine has been the focus of concern in Western capitals since pro-Russian gunmen overran the region after Moscow annexed mainly Russian Crimea from Ukraine in March. Clashes had also occurred in largely Russian-speaking Odessa, not far from Crimea, but no one had died there before.
Police said three people were shot dead and dozens of others wounded in running battles between people backing Kiev and pro-Russian activists in the port city. Another man died later and a further 31 people were killed when a trade union building was set on fire as fighting continued into the evening, police said.
An initial police report had given the death toll from the fire at 38 but the figure was revised.
President Vladimir Putin's spokesman said Ukrainian forces had fired on civilians from the air in Slaviansk in a "punitive operation" that destroyed an international peace plan. Moscow has tens of thousands of troops massed on the border and claims the right to invade if needed to protect Russian speakers.
The Western-backed government in Kiev said the use of missiles to bring down its helicopters showed Russian forces were already in the town and also that Russian "armed saboteurs" had tried to enter the country overnight, but were pushed back by Ukrainian border troops.
Moscow denies involvement with the rebels and its Security Service said the incursion report was untrue.
Kiev said it was forced to act in eastern Ukraine because Moscow was backing groups there who were "putting civilians in danger, seizing hostages and creating an atmosphere of terror and violence".
Reuters journalists in Slaviansk, the most heavily fortified bastion of pro-Russian separatists in eastern Ukraine, heard shooting and saw one helicopter opening fire before dawn. Later in the day, the city was largely quiet, with shops shut and armed separatists in control of the streets.
Advancing Ukrainian forces in armored vehicles took up positions in the suburbs, but rebels still controlled most of the town of 130,000.
Acting President Oleksander Turchinov said the operation had been complicated by the rebels' use of human shields and had not progressed as quickly as had been hoped.