Islamabad  - At least 50 people were killed and around 200 injured Saturday in an apparent suicide bomb attack on the five-star Marriot hotel in Pakistan's capital Islamabad, witnesses and officials said.

"I saw a vehicle approaching the main entrance of the hotel and then it blew up," an injured witness, Mukhtar Ahmad, told Deutsche Presse-Agentur dpa before he was taken to hospital by ambulance.

A security official, who spoke on condition of anonymity, said the deadly vehicle was a truck loaded with around half-a-ton of explosives.

Islamabad police chief Asghar Raza Gardezi confirmed that 50 people were killed and more than 200 injured. He said death toll could rise drastically as many of the injured are in critical condition.

Aaj news channel reported that seven foreigners were among the dead, while a medical officer at Islamabad's Polyclinic said three injured US and Danish diplomats were being treated there.

Four of the five stories of the hotel caught fire after the blast, which was so huge that it was heard kilometres away.

People were trapped in the gutted building and fire brigade vehicles were trying to extinguish the blaze.

"Two children were seen crying for help from a widow in the top floor. But minutes later, before the fire brigade could start working, smoke and fire spread everywhere. Those children are not sighted any more," a reporter for the DawnNews channel said.

The owner of the hotel, a Mr Hashwani, said many of the guests had been evacuated through the rear entrance but some were still trapped.

The luxury hotel is located in a high security zone, about 500 metres from the residencies of Pakistan's prime minister and president.

The blast created panic at the prime minister's house where Premier Yousaf Raza Gilani was hosting a dinner to break the Ramadan fast. President Asif Ali Zardari and military chief Ishfaq Parvez Kayani were also among the guests, DawnNews reported.

No one has claimed responsibility for the blast, but Taliban militants with their bases in country's tribal areas along Afghan border have carried out a series of suicide bombings across Pakistan over the last 18 months.

The blast came on the same day when the new president Zardari vowed before parliament to "root out terrorism and extremism." (dpa)