Islamabad, Oct 26 : At least 20 people including 18 Pakistani troops died in a blast in the troubled Swat district of the North West Frontier Province (NWFP).

The blast took place at Nawan Killi, about a kilometre from Swat city, Mingora, at around 2:45 p.m. on Thursday. It set off an explosion of ammunition carried inside the vehicle carrying Frontier Constabulary (FC) personnel, triggering bullet fire.

Speaking on the condition of anonymity, a senior police official said that it was probably a suicide attack, in which the bomber rammed his car into the truck carrying around 45 security personnel.

"At least 20 people including 18 security personnel died in the attack and 35 others sustained injuries, nine of them civilians," he added.

Swat Nazim Fazlur Rehman said that 30 people had died in the blast.

The Daily Times quoted an injured FC soldier, Naeem Khan, as saying that he could not say whether it was a suicide attack or a planted bomb.

"The nature of the blast is not clear and it is being ascertained. There was ammunition in the truck which caused the damage," Interior Ministry spokesman, Javed Cheema, said.

Maulana Fazlullah, a powerful rebel cleric who advocates jihad and Taliban-style rule in the region, went underground after the attack on the army convoy, the Online quoted sources, as saying.

Fazlullah, who used to run an illegal FM radio station in Swat, had earlier announced after consultations with clerics that he would go to Kohistan.

Sources said that he was not in Kohistan and was believed to be hiding in Swat’s Matta area.

A close aide of Fazlullah said that the cleric’s supporters were not behind the blast.

"We have nothing to do with the blast and our talks with the Chief Minister are continuing. The blast is the planning of those who want to destroy the law and order situation in Swat," said Maulana Shah Dauran.

Meanwhile, Swat residents have started fleeing the area due to the heavy deployment of security forces. The security forces are assisting the local administration to improve law and order in the area.

All markets and bazaars were closed after the blast and police sealed the exit and entry points of the city.

Earlier, the army announced on Wednesday that it was sending in 2,500 additional troops to Swat to maintain law and order.

Fazlullah warned in a speech the same day that his supporters would retaliate if security forces attacked them. (ANI)