An Islamist insurgency in the rugged, thinly populated Sinai 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.
The militant group staging the insurgency pledged allegiance to Islamic State in 2014 and adopted the name Sinai Province. It is blamed for killing hundreds of Egyptian soldiers and police since then.
"An armed group of terrorist elements attacked a checkpoint in North Sinai on Thursday night using four-wheel-drives rigged with explosives," the military said in a statement that put the death toll at eight soldiers and three attackers.
Medical sources said four more bodies were found on Friday, bringing the toll to 12 out of the checkpoint's 31 soldiers. Twelve soldiers were wounded and one was missing.
Islamic State said it had killed 15 soldiers, destroyed two armored vehicles and taken weapons from the checkpoint before blowing it up.
Witnesses said security forces set up extra mobile and static checkpoints in and around Arish city, the capital of North Sinai province, and were searching for the attackers.
Later on Friday, a spokesman for the U.S. State Department condemned the attack, offering condolences to the families of the victims and sympathy for the injured.