The attackers struck almost simultaneously as a crowd of workers streamed out of the huge factory in the northern town of Wah, near Islamabad.

It is one of the country's most sensitive installations.

The Taliban claimed responsibility for the attack, the second to rock Pakistan since Pervez Musharraf resigned as president on Monday, and warned of further blasts if army operations near the Afghan border are not stopped.

Local police chief Nasir Durrani said, ''It's a massive attack.

''Two men apparently blew themselves up outside the factory during a shift change. The bombers were on foot and they exploded themselves less than a minute apart.''

Mr Durrani said 57 people were confirmed dead and about 70 wounded.

The charred body of a bearded man, believed to be one of the bombers, lay on the road outside one of the gates. A severed leg, abandoned shoes and several mangled bicycles were scattered nearby.

Dozens of troops, police and military rescue workers in orange jackets milled around the scene.

Prime Minister Yousuf Raza Gilani strongly condemned the attack and ''directed the authorities to make efforts to expose the hidden hands behind the incident''.

Tehreek-e-Taliban Pakistan spokesman Maulvi Omar claimed responsibility for the bombing.

''Our bombers carried out today's attack. It is in reaction to military operations in Swat and Bajaur,'' he said.

''Similar attacks will be carried out in other cities of Pakistan including Lahore, Islamabad and Rawalpindi.''

Tehreek-e-Taliban is the umbrella group for the country's Taliban militants.

The Pakistani Ordnance Factories at Wah are a cluster of about 20 industrial units producing artillery, tank and anti-aircraft ammunition for the Pakistani armed forces. The factories employ between 25,000 and 30,000 workers.

It adjoins the historic city of Taxila where thousands of tourists come every year to visit ancient Buddhist ruins.

Factory worker Riaz Hussain said most of the victims were labourers who were joining the afternoon shift.

''I was working in the factory when I heard one blast and then another. They were huge,'' Mr Hussain said.

''Security people then immediately surrounded the place and we were not allowed to go outside.''

A rescue service official said a fleet of about 25 ambulances was needed to ferry the wounded to hospital.

The blasts came two days after a suicide bomber attacked a hospital in the north-western town of Dera Ismail Khan, killing 30 people.

Pakistani forces have been fighting for nearly two weeks with Taliban militants in Bajaur, a tribal region bordering Afghanistan. AFP