Three Egyptian policemen and a soldier were injured near the Sinai town of Sheikh Zuwaid on Saturday when armed men fired a rocket-propelled grenade at their convoy during an operation against militants following the killing of 16 border guards.

The attack took place while the police were on their way back to the city of el-Arish after arresting two suspects at the border town of Rafah, a security source said.

"The forces were entrapped by extremists when they were returning from a raid in Rafah ... one armored vehicle was targeted by a rocket," the source said. They could not identify or pursue the attackers, he added.

Disorder has been spreading in turbulent north Sinai, a region awash with guns that has felt neglected by the central government since the overthrow of President Hosni Mubarak last year in a popular uprising. Mubarak's government had worked closely with Israel to keep the border region under control.

Egypt blamed the August 5 border attack on "Islamic militants" and has launched a joint army-police operation that has raided militant hideouts, arrested their members and seized weapons.

A Sinai-based Islamist militant organization, the Salafi Jihadi Group, warned the Egyptian army last week that the crackdown on jihadists in the area would force it to fight back.

There have been frequent reports of attacks on police and army checkpoints since the crackdown began, but often without casualties.

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