"An explosion happened inside a post of the resistance in Beit Lahya," said interior ministry spokesman Iyad al-Bozum, in reference to the Ezzedine al-Qassam Brigades, the military wing of Hamas.
The health ministry spokesman in Gaza, Ashraf al-Qudra, said the blast wounded 50 people, five of them in a serious condition, and including children.
Witnesses said the blast took place inside a training camp of the Ezzezdine al-Qassam Brigades in the northern Gaza Strip, where Hamas is the de facto power.
He said the hospital "declared a state of emergency" after the explosion took place.