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.

Former prime minister Nawaz Sharif said this week the government should reject U.S. pressure to fight militants, halt offensives and negotiate peace.

WIDESPREAD DOUBT

Despite widespread doubts about the wisdom of the U.S. alliance, some analysts say the devastation on Saturday will rally public support rather than force a change of tack.

"It will build the resolve of Pakistanis to support their government more," Khalid Aziz, a former top administrator in the northwest who runs a think-tank, wrote in the News newspaper.

President Azif Ali Zardari, who is close to the United States and has called for terrorism to be rooted out, is due to hold talks with U.S. President George W. Bush in New York on Tuesday.

The United States is Pakistan's biggest aid donor and its support will be crucial if Pakistan is to avoid bankruptcy.

Moody's Investor Services downgraded the outlook on Pakistani debt to negative on Tuesday.

The rupee PKR= has lost 21.2 percent against the dollar this year and is trading around all-time lows. The main stock index has lost about 35 percent this year and had been propped up by a floor since last month.

Compounding a sense of crisis, gunmen kidnapped Afghanistan's top diplomat to Pakistan, Abdul Khaliq Farahi, after killing his driver in an ambush on Monday in Peshawar.

Afghan Defense Minister Abdul Rahim Wardak said on Monday Pakistan and Afghanistan were discussing a joint force to combat militants on both sides of their border. A Pakistani military spokesman said he was not aware of any such proposal. (Additional reporting by Augustine Anthony and Kamran Haider; Editing by Robert Birsel and David Fox)