The Islamist group that has claimed responsibility for bombing the Marriott Hotel in Islamabad was hitherto unheard of, but Pakistani intelligence eavesdroppers heard al Qaeda operatives celebrating the attack.

The group calling itself Fedayeen Islam (Partisans of Islam) made the claim of responsibility for Saturday's suicide truck bombing that killed at least 53 people in a call to an Islamabad-based correspondent for al Arabiya, an Arab news channel.

"It's either new, or it might be a distraction," said a senior intelligence officer, speaking on condition of anonymity.

"What we do know is that there was a lot of celebration among the lower ranks of al Qaeda," he said.

The group issued several demands, including for Pakistan to end cooperation with the United States, Arabiya said.

There are many different militant groups operating in Pakistan.

All of them are anti-American, some more focused against India, while some are militant Sunni Muslim groups that are driven by hatred of Pakistan's Shi'ite Muslim minority.

Some also run with the Taliban in Afghanistan, and some are Pashtun tribesmen in the Pakistani Taliban, who see their own country's government as the enemy.

They all, to a large degree, share al Qaeda's world view.

Al Qaeda's strategic objective, according to senior government officials, is the destabilisation of Pakistan, a nuclear-armed Muslim nation of 170 million people allied to the United States.

Until former president and army chief Pervez Musharraf sided with the United States in a global war on terrorism after al Qaeda's attacks on Sept. 11, 2001, Pakistan's military regarded some groups operating in India and Afghanistan as covert assets.

But after Musharraf's foreign policy U-turn, many jihadi groups, egged on by al Qaeda, turned against the Pakistani state, and there were several assassination attempts against him. Over the past few years, the authorities moved against some groups but left others relatively untouched, and there has been a splintering, coalescing and morphing of different groups, and some forged links with the al Qaeda network operating from the Pakistan-Afghanistan border region.

There has been speculation in the media that Qari Saifullah Akhtar, the leader of an al Qaeda and Taliban-linked group called Harkatul Jehadul Islami, could be involved in the Marriott bombing.

That attack was similar in some ways to others his faction is suspected of carrying out. (Editing by David Fogarty)