PESHAWAR, Pakistan - Pakistani police on Sunday arrested pro-Taliban cleric Sufi Muhammad, who brokered a peace deal between the government and militants in the Swat Valley that has since faltered.

Muhammad, father-in-law of Swat Taliban leader Maulana Fazlullah, negotiated a truce with the government in February that imposed Shariah, or Islamic, law in the valley in exchange for an end to two years of fighting. But it was widely seen as an acquiescence to Taliban control of the area.

The deal collapsed in April when the Taliban advanced into neighbouring districts, triggering a military offensive that prompted a spree of retaliatory attacks by militants in the northwest and beyond.

Mian Iftikhar, information minister for the North West Frontier Province, said Muhammad was arrested for encouraging violence and terrorism.

"Instead of keeping his promises by taking steps for the sake of peace, and speaking out against terrorism, he did not utter a single word against terrorists," Iftikhar said in a news conference in Peshawar, adding that the cleric's stance "encouraged terrorism. It encouraged violence."

The military offensive in the Swat Valley and surrounding areas led some 2 million people to flee their homes and take refuge in camps and with relatives across the country. In the last two weeks, hundreds of thousands have been returning home as the offensive winds down, although sporadic fighting persists.

He accused Muhammad of "again preparing to get more people killed" and said: "We cannot let it happen. The price we have paid for the sake of peace, we cannot allow any person to disturb the peace."

The minister said Muhammad would be investigated regarding his role as mediator between the government and the Taliban, and that a case would then be made based on that investigation.

Muhammad's son Azmat Ullah, 12, told The Associated Press that police arrived at his home in Sethi Town on the outskirts of Peshawar in four vans and took away his father and three brothers.

"My father and brothers went with them without offering any resistance," Ullah said.

One witness, local resident Mohammad Arif, said police fired a shot in the air to disperse a crowd that had gathered during the operation to arrest the cleric.

Muhammad leads a pro-Taliban group known as the Tehrik Nifaz-e-Shariat Mohammedi, or the Movement for the Enforcement of Islamic Law. He was jailed in 2002 but was freed last year after he renounced violence.

Muhammad himself was not in control of armed militants in Swat, and the Taliban's ability to bounce back after the recent military offensive against them will depend more on their leaders, including the cleric's son-in-law. Despite occasional rumours to the contrary, none has been captured or known to have been killed.

But his arrest could be an indication that the government will no longer negotiate with the Taliban -- a position likely to please the U.S., which is looking for signs Pakistan is serious about cracking down on militants.

Violence continues to plague the country's northwest. The army said in the past 24 hours a local Taliban commander was killed during clashes in the Swat Valley, while a roadside bomb in the neighbouring Bajur tribal region killed two soldiers.

Two intelligence officials, who spoke on condition of anonymity because they were not authorized to release the information to the media, also said militants kidnapped two security personnel in North Waziristan, and that another six soldiers were wounded by a roadside bomb as they gave chase to the kidnappers.

Separately, government official Aslam Khan said a woman and a child were killed when a mortar hit their home in the Mohmand tribal area. Khan said it was not immediately clear who had fired the mortar.

Earlier Sunday, police said they had arrested former lawmaker Shah Abdul Aziz and a suspected Taliban militant in connection with the beheading of Polish geologist Piotr Stanczak, who was kidnapped near the Afghan border last September and beheaded.

Investigator Malik Tariq Awan told the AP that the two were taken into custody a month ago.

Awan said Aziz, a member of a pro-Taliban religious party who was elected to parliament's lower house in 2002, is believed to have plotted the geologist's abduction.

Stanczak's beheading -- which was shown in a video that surfaced in February -- was the first of a westerner in Pakistan since Wall Street Journal reporter Daniel Pearl's in 2002.

The other man detained was identified as suspected militant Ata Ullah, who Awan admitted to being involved in the beheading before an anti-terrorism court in Rawalpindi.

The two are to appear in court on Monday, when police will seek to have their custody order extended.