At least five people were injured when a car bomb exploded near an intelligence building of the Egyptian army in the city of Ismaliyha, security sources and state media have said.
The force of the Saturday afternoon blast, which happened northeast of Cairo, caused part of the military intelligence building's wall to collapse, one of the sources said. Sources said another undetonated car bomb was found while security personnel were combing the area.
Other witnesses said they heard loud gunfire around the time of the explosion.
A surge in violence against army and police forces has hit the largely lawless Sinai peninsula, adjoining the Palestinian Gaza strip and Israel.
Armed groups mostly based in North Sinai have escalated attacks on security targets since July 3, when the army deposed President Mohamed Morsi of the Muslim Brotherhood and installed a new government in response to mass protests against his rule.
The Brotherhood, now outlawed in Egypt, called his overthrow a coup, whereas the army says it was responding to the will of the people. Security forces have killed hundreds of the movement's supporters since August, when they crushed two protest camps.
The Egyptian army launched a large military operation in Sinai in September to clear it of armed fighters, seizing
weapons including anti-aircraft missiles and hand grenades.