At least 235 people have been killed after dozens of militants set off a suicide bomb and opened fire at a mosque in Egypt's restive northern Sinai province in the worst religious extremism attack the country has ever faced.

The attack on Friday targeted supporters of the security forces attending prayers there, two eyewitnesses and a security source said.

Four off-road vehicles arrived at the majority-Sufi Al Rawdah mosque in Bir al-Abed, near Arish city, during the sermon section of prayers. Survivors told local media that those who were not killed in the blast and tried to run away were gunned down and the gunmen set vehicles on fire to block access to the building.

"They were shooting at people as they left the mosque," a local man whose relatives were in the mosque during the attack told Reuters. "They were shooting at the ambulances too."

Unverified pictures on social media showed dozens of bodies, clothes soaked with blood, laid out on the mosque's floor. Others showed ambulances lining up at the mosque's entrance to take the wounded to nearby hospitals.

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While no claim of responsibility has yet been made, Egypt has been fighting several extremist groups in the Sinai since 2014, including an Isis-affiliated insurgency. Extremism there has flourished amid the chaos that has engulfed the country since the 2011 revolution.

In comments on Friday evening Egyptian President Abdul Fattah al Sisi vowed the country would respond to the attack with "brute force".

Friday's incident comes after a September attack on a police convoy near the town, which killed 18 people. It was claimed by the Sinai Group, which pledged allegiance to Isis in 2014.

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While the majority of the Sinai Group’s operations primarily target the police, army and other security forces, the Isis affiliate has also carried out suicide bomb attacks and executions of Sufi Muslims, whose interpretation of Islam they consider heretical.

Outside of the Sinai, Egypt’s 10 per cent Christian minority has been a frequent target for extremists, killing dozens of people in attacks on churches.

Condolences have poured in from around the world for the victims of the unprecedented attack. "Appalled by the sickening attack on a mosque in North Sinai. Condolences with all those in Egypt affected by this evil and cowardly act," UK Prime Minister Theresa May tweeted.

President Sisi convened an emergency security cabinet meeting in Cairo to discuss the situation and declared three days of national mourning.

There is growing public anger in Egypt at the government’s seemingly inability to prevent suicide bombings and other terror attacks.