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 US 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 US-based chain and a favorite haunt of diplomats and wealthy Pakistanis.

Pakistani policemen and onlookers gather as a cloud of smoke billows from the burning Marriott hotel. [Agencies]

Internal security in 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 said.

Four foreigners were killed including the Czech ambassador, his Vietnamese partner and two members of the US armed forces assigned to the US 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 defense studies.

An al Qaeda video, released to mark the seventh anniversary of the September 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 destabilize the country. They want to destabilize 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 US 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 meters 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.