Security forces backed by helicopters attacked a stronghold of a militant cleric in northwestern Pakistan on Friday, trading fire with his supporters near the scene of a suicide attack that killed 20 people, police said.

The fighting broke out in the village of Imam Dheri where the cleric, Maulana Fazlullah, has a sprawling seminary. Earlier this week, some 2,500 paramilitary troops were deployed in the surrounding district of Swat to combat his militant supporters.

Pakistan army spokesman Maj. Gen. Waheed Arshad said army troops were sent to the region as reserves to help local authorities maintain law and order, if requested.

"It is not an army operation," Arshad told Pakistan's Dawn television news, adding that the army had provided helicopters to provide surveillance and cover for the security personnel.

Militants in the village and security forces fought with rocket-propelled grenades, mortars and other weapons across the rushing Swat River, witnesses said. Hundreds of residents fled, local shop-owner Rahman Khan said.

Residents said they saw four helicopters hovering over the area and could hear loud explosions from heavy weapons fire. Mohammed Zubair, 35, said he saw one of the choppers firing rockets near Fazlullah's house.

"The security forces attacked a building where Maulana Fazlullah had been appearing in recent days to urge his followers to target the Pakistan army, police and other security forces," said Muhib Ullah, a local police official, in the nearby town of Mingora.

Ullah said it was unlikely Fazlullah was inside the seminary. In an FM radio broadcast on Wednesday, Fazlullah reportedly announced he was shifting to neighboring mountainous district, Kohistan.

There was no immediate word on casualties.

Also Friday, militants fired at a helicopter carrying a senior army officer. They missed the target and the helicopter made safe landing in the same area said another local police official, speaking on condition of anonymity because he was not authorized to speak to media.

The army declined comment.

On Thursday, a suicide car bomber hit a truck carrying Frontier Constabulary paramilitary troops through a crowded area of Mingora, the main town in Swat district, killing 19 soldiers and a civilian, and wounding 35.

The devastating attack underlined the worsening security situation in Pakistan, particularly in the conservative region near the border with Afghanistan where militants linked to the Taliban and al-Qaida increasingly hold sway. The rise of militancy in the region has shaken the authority of President Gen. Pervez Musharraf, a key U.S. ally in the war on terror.

Fazlullah's spokesman denied the cleric's involvement in the bombing, saying he wanted peace in the region and only wanted to impose Islamic law.

Swat, until recently regarded as one of Pakistan's main tourist destinations, lies about 30 miles north of the city of Peshawar, the capital of North West Frontier Province.

Fazlullah is the leader of Tehrik Nifaz-e-Sharia Mohammed, or Movement for the Enforcement of Islamic Law, a banned pro-Taliban militant group which sent thousands of volunteers to fight in Afghanistan during the U.S.-led invasion in 2001.

The Mingora blast came a week after the bloody assassination attempt in the southern city of Karachi on ex-Prime Minister Benazir Bhutto, who plans to start traveling elsewhere in Pakistan on Saturday.

Bhutto, whose grand homecoming to Pakistan after an eight-year exile was shattered by a suicide bombing that killed 143 people, is widely seen as a possible partner of Musharraf in fighting extremism if she fares well in upcoming parliamentary elections.

Bhutto is due to go to her hometown of Larkana on Saturday to pay homage at her father's tomb, about 270 miles northwest of Karachi. She also wants to go to Lahore and the capital Islamabad despite fears of another attack.

Bhutto has blamed Islamic militants for last week's attack on her convoy in Karachi, but also accused elements in the government and security services of complicity in assassination plots, demanding international experts be called in to help in the investigation. The government has rejected such a move.