Palestinian officials say dozens of people were injured when a bomb exploded at a training base used by the Islamic militant group Hamas, which rules the territory.

Gaza health official Ashraf al-Kidra said "dozens of people were injured, several of them seriously," without providing precise figures. Witnesses say Thursday's blast sent a pillar of smoke into the sky.

Eyad Bozum, spokesman for the Hamas-run Interior Ministry, said authorities were investigating if the explosion was from a bomb left over from last year's war with Israel or if it was a "work accident." He confirmed the blast took place at a Hamas training base.

The Israeli military said it was not involved.

Last summer's war left some 2,200 people dead on the Palestinian side and 72 on the Israeli side.