Pro-Russian separatists kill four Ukrainian soldiers during Sloviansk ambush

Pro-Russian separatists have killed four Ukrainian soldiers and injured 30 others during clashes near the city of Sloviansk, dealing a significant blow to the country's renewed anti-terrorist operation.

The 90-minute ambush also resulted in fatalities and casualties among the insurgents, who have occupied government buildings in at least nine eastern Ukrainian cities since last month. They have also promised to hold a referendum on 11 May for the annexation of Donetsk.

In addition, the pro-Russian activists, armed with heavy machine guns, shot down a Ukrainian Mil Mi-24 helicopter at a check point in Sloviansk, making it the third Ukrainian helicopter to have been downed over the heavily guarded city in the past four days.

The Ukrainian Defence Ministry said in a statement that the gunship was gunned down while on combat duty without any casualties, as the helicopter fell into a nearby river.

"The pilots are undamaged. The operation team of the anti-terror centre evacuated them to the camp," the statement said.

Ukrainian Interior Minister Arsen Avakov said the terrorists are opening fire against Ukrainian special units with heavy weaponry, including grenade and rocket launchers, although he claimed that the operation is going as planned.

"The aims of [the] anti-terrorist operation and the demands to the terrorist are simple: to discharge hostages and residential buildings, to drop the arms and to renew the ordinary city property," Avakov said.

The latest casualties follow the fatal fire in the southern city of Odessa on 2 May, which claimed the lives of more than 40 people, mostly pro-Russian separatists.

More than 80 people have been killed since the Kiev Government launched military operations to regain control in the east, Russian media reported.