The death toll from a militant attack on an Egyptian mosque has been raised to 305, including 27 children, the country's chief prosecutor has said.

Nabil Sadeq said the attack in the Sinai Peninsula on Friday also left 126 people wounded.

He said it was carried out by 25 to 30 militants who arrived at the mosque in the small town of Bir al-Abd in five all-terrain vehicles.

He says the militants stationed themselves at the mosque's main door and 12 windows before opening fire on worshippers inside. They also torched seven cars parked outside.

There was no claim of responsibility for the attack, the deadliest by Islamic extremists in Egypt's modern history.

The mosque is frequented by Sufi Muslims, a mystic school of Islam that militants consider heretic.