Militants bombed a Sufi mosque and fired on worshippers in the volatile Sinai Peninsula during Friday prayers, Egyptian officials said, killing at least 155 people.

The extremists attacked the al-Rawdah mosque in the town of Bir al-Abd, 40 km (25 miles) from the North Sinai provincial capital of el-Arish, opening fire from four off-road vehicles on worshippers inside during the sermon, three police officers said as quoted by Associated Press. 

Egypt’s state news agency reported the casualty toll, citing “official sources,” revising it upward several times following the officials’ initial reports.

The attack was the largest single targeting of Egyptian civilians and the first on a large mosque congregation since Daesh began its campaign of violence against the state following the military’s 2013 overthrow of an elected but divisive Islamist president.

Cairo’s international airport boosted security following the attack, with more troopers and forces seen patrolling passenger halls, conducting searches and manning checkpoints at airport approaches.

Hundreds of soldiers and militants have been killed in the conflict, although exact numbers are unclear as journalists and independent investigators are banned from the area.