* U.S., Pakistani intelligence suspect al Qaeda
* Death toll at 53, includes Czech ambassador
* Pakistani prime minister sees threat to democracy
* Some expatriates consider leaving
By Aftab Borka
ISLAMABAD, Sept 21 (Reuters) - A suicide bomb attack that killed 53 people at the Marriott Hotel in Pakistan's capital bore the hallmarks of an operation by al Qaeda or an affiliate, Pakistani and U.S. intelligence officials said on Sunday.
Teams combing the burnt shell of the hotel found more charred bodies after the blast on Saturday evening ignited a blaze that swept through the hotel, part of a U.S.-based chain and a favourite haunt of diplomats and wealthy Pakistanis.
Internal security in nuclear-armed Pakistan, a country vital to the war against al Qaeda and other Islamist militant groups, has deteriorated alarmingly over the past two years.
"The sophistication of the blast shows it's the work of al Qaeda," a Pakistani intelligence officer told Reuters.
Four foreigners were killed including the Czech ambassador, his Vietnamese partner and two members of the U.S. armed forces assigned to the U.S. embassy. Denmark's security service said one of their staff, attached to the Danish mission in the capital, was missing, presumed dead.
An American State Department employee was also missing, a spokesman said.
The Interior Ministry said 266 people were wounded, 11 of them foreigners, after the bomber blew up a truck packed with 600 kg (1,320 lb) of explosives including artillery shells, mortar bombs and shrapnel.
Pakistan's army is in the midst of an offensive against al Qaeda and Taliban fighters in the Bajaur region on the Afghan border. The United States has intensified attacks on militants on the Pakistani side of the border, alienating many Pakistanis.
Militants have launched bomb attacks, most on security forces in the northwest, in retaliation. Security analysts said the militants wanted to show they could strike anywhere unless the government changed its policies.
"(It) underscores the ability of these groups to really challenge the authority of the state in the heart of the capital," said Riffat Hussein, a professor of defence studies.
An al Qaeda video, released to mark the seventh anniversary of the Sept 11, 2001, attacks in the United States, included a call for militants in Pakistan to step up their fight.
ATTEMPT TO DESTABILISE PAKISTAN
"They want to destabilise the country. They want to destabilise democracy. They want to destroy the country economically," Prime Minister Yousaf Raza Gilani told reporters.
A civilian government led by Gilani was sworn in six months ago after nine years of rule by former army chief and firm U.S. ally Pervez Musharraf. It is also facing an economy on the verge of collapse.
The attack will be a big blow for foreign investment and will lead to further weakening of the rupee which is already trading at a record low, dealers and analysts said.
The attack was staged hours after new President Asif Ali Zardari, widower of assassinated former prime minister Benazir Bhutto, made his first address to parliament, a few hundred metres from the hotel, calling for terrorism to be rooted out.
Zardari called the bombing cowardly.
"This is an epidemic, a cancer in Pakistan which we will root out," he said in a televised address.
Army chief General Ashfaq Kayani said the army stood with the nation in its resolve to defeat the forces of extremism and terrorism.
Saturday's attack was the worst in the capital.
The blast left a crater 24 feet (7 metres) deep and 59 feet (18 metres) wide, ministry official Rehman Malik told a news conference.
Malik showed security camera footage from the front of the hotel, which had been bombed twice before, showing a truck trying unsuccessfully to force its way through security barriers.
A small blast could be seen going off in the truck cab, apparently as the bomber blew himself up with a grenade, which started a fire. Minutes, later, after a guard tried to put out the fire with an extinguisher, the truck blew up.
Flames and smoke poured out of the 290-room, five-storey hotel located in a high security zone. Dozens of cars were destroyed and windows shattered hundreds of metres away.
Survivors said hotel security men had warned guests to move to the back of the building shortly before the bomb went off.
Most people managed to flee from the fire before it spread but a Reuters photographer saw a body lying on a top floor balcony on Sunday morning.
Malik suggested the investigation would end up pointing to al Qaeda and Taliban militants based in the Federally Administered Tribal Areas (FATA) on the Afghan border
"In previous attacks, all roads led to FATA," he said.
The United States was ready to assist with the investigation if requested, said U.S. embassy spokesman Lou Fintor.
Some Islamabad-based expatriates were considering leaving, after shrugging off smaller blasts in the city.
"I'll be speaking to my boss tomorrow," said Steve, a Briton working in Islamabad who did not want to give his full name.
Zardari, who won a presidential election this month, left for the United States on Sunday and is scheduled to meet President George W. Bush in New York on Tuesday before the U.N. General Assembly. (Additional reporting by Robert Birsel, Zeeshan Haider, Kamran Haider, Augustine Anthony and Sahar Ahmed; Writing by Simon Cameron-Moore; editing by Matthew Jones/Keith Weir)