CAIRO — Egypt said its warplanes struck militant targets in North Sinai on Friday night, destroying weapons dumps as ground forces raided militant hide-outs, as part of a major offensive against the Islamic State in one of the group’s most enduring Middle East strongholds.

President Abdel Fattah-el Sisi, who is standing for re-election next month, ordered the offensive after Islamic State militants killed at least 311 people in a ruthless gun-and-bomb assault on a mosque in North Sinai on Nov. 24, in Egypt’s deadliest terrorist attack. Soon after, Mr. Sisi set a three-month deadline for the army to defeat the militants. The operation began on Friday morning.

But although the military prepared the Egyptian public for large-scale casualties, with hundreds of hospital beds set aside and doctors recalled from leave, it provided scant detail on the operation. The main news release, issued in video form by the military, featured dramatic imagery and music but no information on the scale or the goals of an operation that the military has billed as its biggest push against the Islamic State in years.