Six Ukrainian soldiers were killed when two military helicopters crashed on a combat mission in the country's east, state investigators said Thursday.

The crashes took place Tuesday in the Kramatorsk district of the eastern Donetsk region. the State Bureau of Investigation (SBI) did not give details on what happened.

"Two Mi-8 military helicopters crashed during a combat mission. Six servicemen of the Armed Forces of Ukraine died," the SBI said in a statement.

It said a pre-trial investigation by the Security Intelligence Service would provisionally focus on possible safety breaches during or in preparation for the flight, which would be classed as a criminal offence.

The probe will also look into "possible sabotage or destruction of the helicopters by the enemy", the SBI said.

It said investigators would examine data from flight recorders and check the helicopters' technical condition.

Mi-8 military helicopters are being used by Ukraine in combat missions, although the Soviet-designed aircraft are generally used for transport. Each helicopter has a crew of three.

Many of Ukraine's Mi-8 helicopters are decades old, but it has received donations of more modern attack helicopters from allies.