KIEV (AFP) - Nine Ukrainian servicemen died on Tuesday when pro-Russian insurgents shot down an army helicopter in the separatist east just a day after a top militia leader ordered a temporary ceasefire.

"There were nine people on board the helicopter. According to preliminary information... everyone on board died," Ukrainian defence spokesman Vladyslav Seleznyov wrote in a Facebook post.

Army officials said the Mi-8 helicopter was downed outside the rebel stronghold city of Slavyansk that has absorbed some of the heaviest fighting in the 11-week insurgency in Ukraine's Russified eastern rustbelt.

The Ukrainian army earlier reported fighting outside the city despite a temporary ceasefire being ordered by a top rebel commander on Monday evening.

AFP reporters near the city could also hear echoes of artillery fire about 90 minutes after the helicopter had been downed.

Pro-Russian rebels killed 12 Ukrainian soldiers when they downed another Mi-8 helicopter outside the same city on May 29.

Seleznyov said the rebels hit the chopper with a missile fired from a portable air defence system - a type of heavy weapon that Kiev has accused Russia of covertly supplying to the insurgents throughout the campaign.

He added that the helicopter was returning to its base after delivering special monitoring equipment to an unspecified location in the conflict zone.

"After launching the missile, the rebels vanished in the nearby village of Bylbasivka." The prominent head of the self-declared Donetsk People's Republic on Monday unexpectedly reversed his firm rejection of President Petro Poroshenko's earlier peace overtures by agreeing to a ceasefire that would last until Friday morning.

Poroshenko ordered his forces to halt fire for a week last Friday in an effort to resolve the worst Ukraine's worst crisis since its independence from the Soviet Union in 1991.

President Vladimir Putin followed that up on Tuesday by asking senators to rescind their March 1 authorisation for Kremlin forces to occupy parts of Ukraine as part of a self-proclaimed effort to "protect" ethnic Russians living in the east.

The fighting has now claimed the lives of around 390 civilians and fighters on both sides.