ISMAILIA, Egypt (Reuters) - Armed men seized a military vehicle on Sunday in the city of al-Arish in Egypt's Sinai, where the Egyptian army has launched a security crackdown against Islamist militants.

The armed men riding a pick-up truck stopped the army car in al-Arish, the administrative center of North Sinai, forced out an officer and a soldier then drove into the desert.

Disorder has spread in Sinai since former President Hosni Mubarak was ousted in a popular uprising last year, with Islamist militants stepping up attacks on security forces and the Israeli border. Egypt's new President, Mohamed Mursi, has vowed to restore order.

Egyptian forces began two months ago their biggest security crackdown in decades after militants killed 16 border guards in the deadliest attack there since Egypt's 1973 war with Israel.

But efforts to impose central authority in the lawless desert region are complicated by the indigenous Bedouin population's ingrained hostility to the government in Cairo.

(Reporting by Yousri Mohamed, editing by Diana Abdallah)