UPDATE 1-Pakistani spies hear al Qaeda celebrating blast

(Adds details, background)

By Simon Cameron-Moore

ISLAMABAD, Sept 23 (Reuters) - An Islamist group that claimed responsibility for bombing the Marriott Hotel in Islamabad had not previously been heard of, but Pakistani intelligence eavesdroppers heard al Qaeda operatives celebrating the attack.

The suicide truck bomb that killed at least 53 people and gutted the hotel on Saturday has raised fresh fears about worsening security in Pakistan, a nuclear-armed key U.S. ally.

A group calling itself Fedayeen Islam (Partisans of Islam) claimed responsibility 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. "What we do know is that there was a lot of celebration among the lower ranks of al Qaeda."

The group issued several demands, including that Pakistan ends cooperation with the United States, Arabiya said.

Such calls will fuel worry among many Pakistanis who say the alliance with the United States incites militant violence and Pakistan should not be fighting "America's war".

The army is embroiled in an offensive against militants in the Bajaur region on the Afghan border, while the United States has intensified attacks on militants on the Pakistani side of the border, which has infuriated the Pakistani army.