Security officials say the Pakistani army has started court martial proceedings against a brigadier and four other officers over suspected ties to a banned extremist group that has called for ousting the U.S.-backed government.

Brig. Ali Khan and the four officers were detained in May, 2011 on charges of links to the Hizb-ut-Tahrir group. Khan was working at the army's headquarters near the capital Islamabad at the time of his arrest.

The group is banned because of its extremist propaganda. It is seeking to re-establish the Islamic caliphate that once ruled much of the Muslim world.

Two senior security officers said on Saturday the court martial started recently but did not provide any details. They spoke on condition of anonymity due to the sensitivity of the issue.