Suspected Islamic State gunmen killed 12 members of Egypt's military in North Sinai province and wounded six more in an attack on a checkpoint on Friday, the military said, adding that it killed 15 militants in return.
The incident took place 40 km (25 miles) from the town of Bir al-Abd, making it the first major attack in the central Sinai area, which had so far escaped the militant Islamists' campaign.
It was carried out using assault rifles and some heavier weapons, medical and security sources said. The wounded were taken to hospital in al-Arish.
"An armed group of terrorist elements attacked a security checkpoint in North Sinai this morning using four-wheel drives and were immediately engaged. Our forces killed 15 terrorists," the military said in a statement. "The clashes led to the martyrdom of 12 and injury of six armed forces heroes."
An Islamist insurgency in the rugged and thinly populated Sinai Peninsula gained pace after the Egyptian military overthrew President Mohamed Mursi of the Muslim Brotherhood, Egypt's oldest Islamist movement, in mid-2013 following mass protests against his rule.
The militant group staging the insurgency pledged allegiance to Islamic State in 2014 and adopted the name Sinai Province. It is blamed for the killing of hundreds of soldiers and policemen, and has started to target Western targets within Egypt.
President Abdel Fattah al-Sisi, the former military chief who led the overthrow of Mursi, describes Islamist militancy as an existential threat to Egypt, an ally of the United States. Islamic State controls large parts of Iraq and Syria and has a strong presence in Libya, which borders Egypt.