Pakistan has appointed a new chief for the Inter-Services Intelligence (ISI) Directorate, months after US officials had questioned the reliability of the military's premier spy agency in the war against terrorism.

A military statement issued around midnight on Monday announced a major overhaul of Pakistan's top brass by army chief General Ashfaq Kayani.

The reshuffle will be closely scrutinised by the intelligence community in the United States and neighbours in the region.

A Sept. 20 suicide bomb attack that killed 55 people at the Marriott Hotel in Islamabad underscored concern al Qaeda-linked militants and Taliban fighters could destabilise the nuclear-armed Muslim nation of 170 million people.

Lieutenant-General Ahmed Shujaa Pasha, formerly head of military operations, has been appointed director-general of the ISI, replacing Lieutenant-General Nadeem Taj.

Often referred to by critics as a 'state within a state', the ISI is feared by neighbouring Afghanistan and India, as well as Pakistan's civilian politicians whose governments have been overthrown by military coups.

The ISI had helped the United States eliminate hundreds of al Qaeda fighters in the years after the 2001 attacks on New York and Washington, but US officials have been concerned about diminishing returns from the alliance.

US fears some ISI agents might be playing a double-game supporting Islamist militants came to a head after a suicide bomber killed 58 people outside India's embassy in Kabul in July. - Reuters