Egyptian security officials say militants have opened fire on a police checkpoint in the Sinai peninsula, killing two policemen and injuring a third. The officials say police were manning a checkpoint near the airport of the city of al-Arish early Sunday when militants armed with automatic rifles sprayed them with bullets. Two policemen were shot dead on the spot, one hit in the chest and another in the head. The militants escaped during an exchange of fire. The officials spoke on condition of anonymity because they were not authorized to talk to the media. Islamist militants have stepped up their activity in Sinai, taking advantage of a security vacuum caused by a thin police presence following the uprising that ousted Hosni Mubarak last year.