Many soldiers are reported killed or kidnapped

At least eight soldiers died in the raid and 15 escaped, the army says. The whereabouts of another 25 are unknown.

A Taleban spokesman told BBC Urdu that 16 troops had been killed and another 12 captured during the fighting. Two Taleban died in the fighting, he said.

South Waziristan is a known stronghold of pro-Taleban and al-Qaeda militants.

The region has been at the centre of fighting between the army and the militants in recent months.

Gunfire

"About 200 militants charged the fort from four sides," army spokesman Maj Gen Athar Abbas said.

"They broke through the fort's wall with rockets."

Local officials and other reports suggest the number of militants may have been nearer to 1,000.

Observers say this is the first time that militants have captured a fort in Pakistan.

The army says that up to 40 attackers were killed in the fighting, something the tribal fighters deny.

Taleban spokesman in South Waziristan Maulvi Umar told BBC Urdu service that two fighters had been killed.

There is no independent confirmation of these figures.

Officials said troops at the fort came under rocket and automatic weapons attack from militants on Tuesday night.

Soldiers returned fire and the battle went on until early on Wednesday morning. People in the Sararogha area told the BBC Urdu service the exchange of fire went on for four hours.

There are many forts in South Waziristan

They said the militants entered the remote military outpost and started shifting weapons and troops they had captured out of the fort.

It is unclear whether the militants are still inside the fort.

Correspondents say that Sararogha Fort dates back to the British colonial period.

It is one of several such posts located along South Waziristan's mountainous border with Afghanistan.

Military personnel at the base monitor and patrol the frontier which it is claimed is used by militants to transport weapons into Afghanistan.

Militant leader

Correspondents say the Sararogha area is a stronghold of pro-Taleban militant leader Baitullah Mehsud.

He is accused by the government of being behind the assassination of opposition leader Benazir Bhutto in Rawalpindi on 27 December and a wave of other bombings in recent months.

Meanwhile, the political administration of South Waziristan agency, based in neighbouring North West Frontier Province, has called a council of the Mehsud tribe to try to resolve the issue.

The BBC's Barbara Plett in Islamabad says that in recent weeks there has been an increase in skirmishes on Mehsud tribal lands.

Sources say the government is considering an economic blockade of the tribe.