Dozens of people have been killed in a fire and clashes in the port city of Odessa, as Ukraine’s bloody conflict spread from the east to the south.

A trade union building was set on fire, reportedly after pro-Russian activists took refuge inside, having been chased by pro-Kyiv demonstrators.

At least 31 people died from carbon monoxide poisoning or after jumping from windows, according to the Ukrainian Interior Ministry.

Clashes have occurred in largely Russian-speaking Odessa before, but this deadly violence was something new.

At least four people were killed and dozens wounded amid running battles between pro-Kyiv and pro-Russian supporters.Some reports put the overall death toll at 43.

Odessa is not far from Crimea, annexed by Russia from Ukraine in March.

Pro-Russian gunmen have overrun eastern Ukraine but early on Friday, Kyiv sent in forces to dislodge the militants from their stronghold in the town of Slovyansk.

The separatists though fought back, downing two Ukrainian military helicopters at the start of the operation.

Deaths were reported on both sides and there was still said to be intense gunfire well into the night.