A massive bomb attack has hit the Marriott Hotel in Pakistan's capital, Islamabad, killing at least 40 people.

The BBC's Barbara Plett, who is at the scene, says the blast created a 20ft (6m) deep crater, and destroyed the entire front section of the hotel.

She says heavy equipment has been brought in to clear the debris so that firefighters can reach the building, which is still engulfed in flames.

Police say they believe the blast was a suspected suicide car bombing.

The owner of the hotel told the BBC that the explosion occurred when a lorry, which was being checked by security staff and sniffer dogs at the hotel entrance, blew up.

The explosion is thought to have been caused by more than a tonne of explosives. Police are warning that the hotel could collapse.

Pakistan has seen a lot of violence over the past year in its battle with Islamist militancy but Islamabad has never seen anything like this before, our correspondent says.

Heavy security

In pictures: Islamabad blast

A huge area of the 290-room hotel remains on fire hours after the explosion.

The BBC's Barbara Plett says the emergency services have been unable to reach the upper floors of the hotel, where more people are feared to be trapped.

A hotel employee, Mohammad Sultan, said he was in the reception when something exploded, forcing him to the ground.

"I don't understand what it was, but it was like the world is finished," he told the Associated Press news agency.

The Marriott is the most prestigious hotel in the capital, and is located near government buildings and diplomatic missions.

It is popular with foreigners and the Pakistani elite. Security there is tight, with guests and vehicles subject to checks.

The Marriott has previously been the target of militants. Last year a suicide bomber killed himself and one other in an attack at the hotel.

Insurgency

The bomb attack comes just hours after Pakistan's newly installed President, Asif Ali Zardari, said he would not allow Pakistan's territory to be violated by terrorists or foreign powers fighting them.

In his first speech to MPs since he replaced Pervez Musharraf in August, he vowed instead to "root out terrorism and extremism wherever and whenever they may rear their ugly heads".

Pakistan has been a key ally of the US in its "war on terror"

Pakistan has been a key ally of the US in its "war on terror", but relations have become strained over tactics.

In recent months, Pakistan has voiced growing disquiet over US raids targeting militants in its territory, launched from neighbouring Afghanistan.

Al-Qaeda and Taleban militants based in Pakistan's north-west tribal region have repeatedly carried out attacks across the border in Afghanistan.

Militants have also carried out waves of attacks in Pakistan in recent years.

Just over a year ago, Pakistani army commandos stormed Islamabad's Red Mosque - also known as Lal Masjid - which had been taken over by pro-Taleban clerics.

The operation brought an end to the bloody siege. But Islamist militants responded with a wave of suicide bombings around the country that killed around 1,000 people.