Armed men have killed five Egyptian soldiers in an attack on a military checkpoint near the Suez Canal city of Ismailiya, security officials said.
Monday's attack took place north of the city, where fighters have repeatedly targeted security forces in recent weeks, the officials said.
In a separate incident, officials said a massive explosion, possibly from a car bomb, hit the security headquarters in a southern Sinai town, killing three people and wounding 47.
The security officials said the attack in the town of al-Tour significantly damaged the four-story building and set off small fires.
They said the death toll was likely to rise as rescue workers and residents search for victims under the rubble.
Fighters, some with al-Qaeda links, have been targeting security forces daily in northern Sinai in what has increasingly become a full-fledged insurgency.
But the southern part of the strategic peninsula, which includes the popular diving resort of Sharm el-Sheikh, has been peaceful.
In yet another security incident, an Amaadi Satellite Earth station in Cairo was hit by an RPG attack on Monday.