A KrAZ truck and a Bohdan bus of the Ukrainian army collided in the army operation zone in eastern Ukraine killing 12 servicemen and injuring another 21, the Ukrainian Interior Ministry said.

Eighteen casualties were reported earlier.

The collision happened on Monday in bad weather during a rotation of volunteers and contract servicemen of the Ukrainian National Guard in the army operation zone.

The injured servicemen were taken to hospital in Artemivsk. They are being given aid, provided with medicines and evacuated to hospitals of Kharkiv and other hospitals of Ukraine.

"A commission comprising representatives of the military prosecution service, the road police, an investigative team of the Ukrainian Interior Ministry and the Ukrainian National Guard command is working on the accident site," the ministry said.

According to Hromadske.tv, another serviceman died of injuries in the small hours of Tuesday and the death toll grew to 13.

It said the road accident happened near the village of Minkivka in the Donetsk region. Kharkiv volunteers reported the admission of eleven soldiers by a local hospital. One soldier had a snipe fracture.

There has been no official confirmation to a higher death toll as of yet.