Ukrainian special forces have launched what they say is phase two of their operation against pro-Russian forces in the east.

They are mounting a full-scale blockade of the city of Slovyansk, according to an official on the presidential staff, in an attempt to prevent weapons or reinforcements coming in from outside.

A Ukrainian army helicopter was hit with rocket and sniper fire at the nearby military airstrip on the outskirts of Kramatorsk, detonating its fuel tank and setting off its on-board munitions. The defence ministry said it was totally destroyed.

In a separate incident police reported on Friday that seven people were injured when a bomb went off at a pro-Ukrainian checkpoint near Odessa, far to the west.

At one point the Russian military exercises that began on Thursday using forces deployed from Volgograd came within one kilometre of the Ukrainian border. Ukraine has warned Russia that if any cross the border they will be “liquidated.”