The army helicopters had targeted two villages south of Sheikh Zuweid, a town near the border with Israel and the Hamas-run Gaza Strip.
An army source said a ground operation was under way with air cover but declined to give further details.
Security sources said government helicopter strikes had killed at least eight armed men and wounded 15 and had been aimed at stores of arms and explosives in the Sinai.
Militant attacks on security forces in the lawless North Sinai region have risen since the army, prompted by mass protests, ousted Egypt's first elected Islamist president, Mohamed Mursi, on July 3.
Also yesterday, an Egyptian military court sentenced 11 supporters of Mr Mursi to life imprisonment on charges of attacking the army.
The men were accused of wounding seven soldiers with birdshot and throwing petrol bombs at military buildings in the city of Suez last month.
On that day, one of the bloodiest in Egypt's modern history, security forces crushed Cairo protest camps demanding Mr Mursi's reinstatement.