PESHAWAR, Pakistan — Security forces opened fire Friday on a stronghold of a militant cleric in northwestern Pakistan, sparking a fierce gunbattle near the scene of a suicide attack on a military truck that killed 20 people, police said.

The fighting broke out at the village of Imam Dheri, where pro-Taliban 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.

"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.

He said he could hear loud bangs, and another police official Anwar Khan reported heavy weapons fire and that a helicopter was flying over the area.

There was no immediate word on casualties.

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

On Thursday, a suicide car bomber hit a truck carrying Frontier Constabulary troops through a crowded area of Mingora, 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 its 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.

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

The 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 136 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 of 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.

Associated Press writers Munir Ahmad in Islamabad and Zarar Khan in Karachi contributed to this report.