Ten Egyptian soldiers were killed and 35 wounded in a car bomb attack near the North Sinai city of El-Arish on Wednesday, a security official said.
The attack was one of the deadliest in the Sinai Peninsula, which is near Israel and the Palestinian-run Gaza Strip, since al Qaeda-inspired militants began stepping up assaults following the army's ouster of Islamist President Mohamed Mursi in July.
The soldiers were traveling in a convoy on the road to the Rafah border crossing with Gaza. There was no immediate claim of responsibility.
A Sinai-based militant group, Ansar Bayt al-Maqdis, said it assassinated a high-ranking security officer in Cairo on Sunday, according to a statement posted on a militant Islamist website. That group has also said it was behind a failed suicide attack on Egypt's interior minister in September.
In a separate incident on Wednesday, three people were wounded in a bomb attack on a security checkpoint in Cairo, state television reported.
Fears are growing that an Islamist insurgency will take hold beyond Sinai.
The violence and the political struggle between Mursi's Muslim Brotherhood and the army-backed government has hit investment and tourism in Egypt, an important U.S. ally in the region.