Hamas has arrested four members of the Fatah's al-Aqsa Martyrs Brigades for firing rockets at Israel or trying to do so, a Hamas member said on Friday.

Four Brigades members "were arrested in Beit Hanun overnight as they prepared to fire rockets in response to Israeli violations" of a three-week-old truce, the Hamas source said on condition of anonymity.

Another three members of the group were detained on Thursday after two rockets were fired at the southern Israeli town of Sderot, the source said.

Brigades spokesman Abu Qussai said the trio was released on Friday, adding that he hoped the others would also be freed "as quickly as possible."

The Brigades, an offshoot of Palestinian president Mahmud Abbas' Fatah party, claimed Thursday's rocket attack, saying it was in retaliation after Israeli troops shot dead one of its members.

A spokesman for the Hamas movement that controls Gaza, Islam Shawan, said "we neither confirm nor deny these arrests."

On Thursday, however, the Brigades condemned the "persecution by dozens of armed Hamas men of our fighters on the way to fire rockets in response to the crimes of the (Israeli) occupation in the West Bank and Gaza."