The international human rights organization Amnesty International has called for an immediate impartial and objective investigation into the shelling of a bus carrying civilians near the town of Volnovakha, Donetsk region.

"The world needs to know the truth about this tragic incident. It must be investigated thoroughly, impartially and independently, as a possible violation of the rules of war," the organization's press service quoted Denis Krivosheev, Deputy Europe and Central Asia Director at Amnesty International, as saying.

According to him, those responsible for this incident "either failed to take steps to protect civilians, as demanded by international humanitarian law, or it was a deliberate act which would constitute a war crime."

As reported, a passenger bus came under fire near the town of Volnovakha, Donetsk region, on Tuesday afternoon. Twelve people were killed and another 17 suffered injuries as the result of the attack.

The Ukrainian military reported that the bus was hit by shells from a BM-21 Grad multiple rocket launcher, located in the center of the rebel-controlled town of Dokuchayevsk.

An inquiry into the accident has been launched under part 3 of Article 258 of the Criminal Code (terrorist attack). The investigation is under the auspices of Donetsk regional prosecutor's office.