A homicide car bomber hit a military truck in northwestern Pakistan on Thursday killing at least 20 people, two days after troops were sent in to quell pro-Taliban militants, official said.

The blast comes a week after a bloody assassination attempt on ex-prime minister Benazir Bhutto in the southern city of Karachi. She plans to start traveling elsewhere in Pakistan on Saturday.

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The latest attack underlined failing security, particularly in conservative regions of Pakistan near the border with Afghanistan where militants linked to the Taliban and al-Qaida increasingly hold sway.

The bomber hit a truck carrying about 45 Frontier Constabulary forces, near the main police station in Swat district, where 2,500 paramilitary troops were deployed this week to counter a militant cleric. The impact of the blast tipped the truck on its side and detonated ammunition inside.

Police officer Ajab Khan said 20 people were killed and 34 wounded, mostly soldiers. Some bystanders in shops and restaurants along the road were also hit.

"It was a huge explosion. Then the truck was on fire. There were flames, smoke and people crying. People were scared to go near because bullets were going off," said Taj Mohammed Khan, 23, a college student who was drinking tea at a nearby roadside restaurant at the time of the blast.

Police said it was a homicide attack, and Fazlur Rehman, the top local official at the scene, said the engine of another vehicle was found near the truck, showing it was a car bomb. It took firefighters several hours to extinguish the burning truck, he said.

There was no immediate claim of responsibility for the attack.

The paramilitary troops were deployed Tuesday to Swat, a mountain valley popular with tourists until violence flared there this summer, to quell Maulana Fazlullah and his militant supporters. Fazlullah has called for Taliban-style rule and holy war against Pakistani authorities.

But his spokesman denied the radical cleric's involvement in Thursday's bombing, saying he wanted peace in the region. The cleric was only trying to impose Shariah, or Islamic law, by punishing criminals, including "murderers, abductors and wine drinkers."

"There is absolutely no need for the army here," spokesman Sirajuddin, who goes by one name, told The Associated Press by phone, saying the deployment of security forces was the source of the problem.

"This happens when the army comes here," he said.

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

As well as marshaling armed militants and enforcing Islamic law, Fazlullah has used an FM radio station to campaign against girls' education and to denounce a recent polio vaccination program as a Western plot to sterilize Muslim children.

The deteriorating security in the valley is part of a broader trend of Islamic militants challenging the government's authority across northwest Pakistan. That has shaken the authority of President Gen. Pervez Musharraf, a key U.S. ally in its war on terror.

Bhutto, whose grand homecoming to Pakistan after an eight-year exile was shattered by a homicide bombing that killed 136 people, is widely seen as a possible partner for Musharraf in fighting extremism if she fares well in upcoming parliamentary elections.

On Thursday, Bhutto announced a plan to travel to her hometown of Larkana on Saturday to pay homage at her father's tomb, about 430 kilometers (270 miles) northwest of Karachi.

She has also indicated that she plans to travel to Lahore and the capital Islamabad, despite her professed fear that she will be attacked again.

She told reporters Thursday that authorities had yet to meet her requests for a vehicle with darkened windows, for her guards to carry guns and for four police cars to escort her car, instead of the current two.

"I should be made to feel secure. I should not be made to feel insecure," she told reporters in Karachi.

Still, police in Larkana said they were confident they could protect her.

"We have received no intelligence report so far suggesting any threat to Benazir Bhutto's life here in her hometown," said Nisar Channa, a senior police officer. Some 300 police were already deployed at the airport or along roads that she might use.

Bhutto has been hunkered down at her heavily guarded Karachi residence for the past week, other than a couple of quick trips out in the city, once to visit victims of the bombing in hospital.

She has said she would avoid mass rallies because of the risk of further bombings, but vowed to meet the public across the country.

Bhutto has blamed Islamic militants for the attack on her, but has also accused elements in the government and the security services of complicity in plots to kill her, demanding international experts be called in to help in the investigation — a call rejected by the government.