An ambush by pro-Russian separatists has killed at least six Ukrainian soldiers, the heaviest loss of life for government forces in a single clash since Kiev sent soldiers to put down a rebellion in the country's east.

Ukraine's defence ministry and state security service on Tuesday said the troops were killed and seven others wounded when their armoured column was ambushed near the town of Kramatorsk, one of several hot spots in the largely Russian-speaking east where the army has had scant success against the rebels.

Al Jazeera's Paul Brennan, reporting from Donetsk, the incident occurred at 13:00 local time when a convoy of military units from the Ukrainian army was ambushed by a group of about 30 people the ministry called "terrorists".

The attackers had taken cover among bushes along a river and used automatic weapons and grenade launchers.

According to the ministry, the first shot was fired from a grenade launcher at the convoy and hit an engine of one of the vehicles.The initial attack killed two and injured three, the ministry said.

All the dead and injured have since been evacuated.

Our correspondent said a pro-Russian website had made the claim that 30 soldiers had died in the attack, but this information could not be independently verified.

Krematorsk is in the northern part of the Donetsk region, which on Sunday declared its independence from Ukraine and sought an alliance with Russia following a vote for self-rule in the area.

The attack comes less than two weeks ahead of Ukraine's presidential elections, slated by many as a means of restoring peace, but something that both Donetsk and Lushank, the second area to declare independence, have said will not be held in the regions.

"The timing of this attack is particularly bad," our correspondent said. "When you have incidents such as what happened this afternoon it really does overshadow the efforts of diplomats and politicians.

Another nine Ukrainian soldiers have been killed since Kiev launched a so-called "anti-terrorist" operation in the east in mid-April, a defence ministry official said.

The campaign has been directed mainly against rebels in the towns of Slovyansk and Mariupol.

The dead included 5 pilots, Defence Minister Mikhailo Koval said, who apparently died when their helicopters were downed by separatist fire.