Separatist rebels in eastern Ukraine say an accidental grenade explosion on a bus has killed one teenager and wounded three others.

Rebel military spokesman Ruslan Yakubov said in a statement carried by the Donetsk News Agency that the grenade was in a rucksack belonging to one of the boys when it went off on Tuesday.

It wasn't immediately clear where the boy obtained the grenade; eastern Ukraine is awash in weapons after four years of fighting between the rebels and Ukrainian forces.

Yakubov says the teen victims are between the ages of 14 and 16 and were heading home from school when the explosion rocked the bus in the city of Debaltseve.

The Ukrainian conflict erupted after Russia's annexation of Crimea and has killed more than 10,000 people since April 2014.