Brig. Ali Khan and four other officers were detained in May, 2011 for suspected 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. Khan's family has denied the allegations against him.
Two senior security officials, speaking on condition of anonymity because of the sensitivity of the issue, said the military trial started recently but did not provide any other details. It was also not clear yet where the trial is taking place and what punishment the officers face, if convicted.
Hizb-ut-Tahrir is banned in Pakistan because of its extremist propaganda. The group also works to re-establish the Islamic caliphate, the administrative structure that once governed much of the Muslim world, and campaigns in Pakistan against the country's alliance with the Unites States in the war on terror.
The group insists it has rejected violence, although observers say it promotes an intolerant mindset that can ultimately lead some followers to embrace militancy.
Western officials have long suspected some Pakistani military officials of having ties to Islamist groups. Those fears spiked after American forces discovered and killed Osama Bin Laden in an army town not far from Islamabad - although the US has found no evidence that senior Pakistani officials knew of Bin Laden's whereabouts.
The Pakistan military has repeatedly denied supporting extremist groups.
Khan's lawyer, retired Col. Inam Rahim, said after the arrest that his client was detained for demanding that someone within the military be held accountable for the covert US Navy SEALs raid that killed Bin Laden in Abbottabad.
