Egyptian officials investigating the massacre of worshippers at a mosque in Sinai say the attackers were carrying the flag of the Islamic State group.

At least 305 people died in the assault, which was launched during Friday prayers and has not yet been claimed by any group.

Egypt’s public prosecutor said there were up to 30 attackers at the scene.

President Abdul Fattah al-Sisi has vowed to respond with "the utmost force”.

The Egyptian military says it has already conducted air strikes on "terrorist” targets.

Egyptian security forces have for years been fighting an Islamist insurgency in the Sinai peninsula, and militants affiliated with so-called Islamic State (IS) have been behind scores of deadly attacks in the desert region.

Friday’s attack in the town of Bir al-Abed is the country’s deadliest in recent memory.

Al-Rawda mosque was bombed and then dozens of gunmen, waiting outside, opened fire on those trying to escape. Some attackers wore masks and military-style uniforms.

The assailants reportedly set parked vehicles on fire in the vicinity to block off access to the building, and fired on ambulances trying to help victims.

Thirty children are among the dead and more than 100 people have been wounded.

"What is happening is an attempt to stop us from our efforts in the fight against terrorism,” Mr Sisi said in a televised address hours after the attack.

"The armed forces and the police will avenge our martyrs and restore security and stability with the utmost force.”

Agencies