Gunmen killed at least three policemen and injured seven in an attack on a checkpoint in the restive northern Sinai on Thursday, Egyptian security sources said.

There was no immediate claim of responsibility for the attack.

An Islamist insurgency in the rugged, thinly populated Sinai Peninsula has gained pace since the military toppled President Mohamed Mursi of the Muslim Brotherhood, Egypt’s oldest Islamist movement, in mid-2013 following mass protests against his rule.

In Thursday’s incident, assailants fired two rocket propelled grenades at the checkpoint south of the city of al-Arish, and three pick-up trucks carrying masked men then drove by and opened fire, security sources said.

The militant group staging the insurgency in Sinai pledged allegiance to ISIS in 2014 and adopted the name Sinai Province. It is blamed for the killing of hundreds of Egyptian soldiers and policemen since then.

In its weekly online magazine Al-Nabaa released on Thursday, ISIS urged members to join other branches of the group active in areas like the Sinai, Libya, Yemen and West Africa if unable to reach its self-declared “caliphate” in Iraq and Syria.

ISIS had previously called on members to carry out attacks in their home countries if unable to enter Iraq or Syria because of tougher security measures in the region aimed at cutting off militant supply lines.