Ansar Bayt Al-Maqdis, Egypt's most active militant group, has sworn allegiance to Islamic State, a statement from Ansar said Monday night.
Ansar had previously told Reuters that it sought inspiration and advice from Islamic State, the radical Al-Qaeda offshoot that has taken over swathes of Iraq and Syria, drawing US-led airstrikes as it tries to remake the map of the Middle East.
"After entrusting God we decided to swear allegiance to the emir of the faithful Abu Bakr Al-Baghdadi, caliph of the Muslims in Syria and Iraq and in other countries," the statement said.
Last week the government declared a three-month state of emergency in parts of North Sinai following attacks that killed at least 33 security personnel in some of the worst anti-state violence since the ouster of Islamist president Mohamed Morsi.
No group has claimed responsibility for the attacks, but similar previous operations have been claimed by Ansar.