A pickup truck packed with explosives blew up at a security checkpoint in northwestern Pakistan, killing at least 30 people and injuring dozens more in an attack possibly intended for a more important target, officials say.

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About 170km away in the volatile region, 24 people were killed after residents of villages tired of a "reign of terror" by Islamic militants foiled a militant kidnap attempt, then were attacked, the military said.

The suicide attack on Saturday occurred on the outskirts of Peshawar on the day Pakistani lawmakers voted for a new president, underscoring the challenges facing a country the US has pressured to crack down on insurgents.

Television footage showed a blast crater one metre deep and destroyed vehicles and pieces of debris scattered across a large area. Officials said many people were trapped under the rubble of two collapsed buildings in a nearby market. Civilians dug frantically with their hands in hopes of finding survivors.

Nasirulmulk Bangash, a top police official in the area, said the vehicle carried at least 150kg of explosives - an amount he called "unprecedented" - and was apparently en route to Peshawar, the capital of the North West Frontier Province.

The amount of explosives indicated the attack was aimed at a higher-value target than the small checkpoint, but might have been tripped up by high security on election day, Bangash said, without speculating on what the intended target might have been.

Peshawar police chief Muhammad Suleman said late Saturday that 30 people died, including five police officers and several who succumbed to their injuries in a hospital or were found lifeless among the rubble.

Death toll likely to rise

He said the toll was likely to rise as rescue teams continued to dig.

Sher Zaman, 15, told The Associated Press from a hospital bed that he was selling fruit from a cart at the market when he heard a large explosion and was knocked down when something hit him in the chest. He said residents quickly gathered and helped transport casualties to hospitals.

No one immediately claimed responsibility for Saturday's blast.

In recent weeks, however, the Pakistani Taliban have said they were to blame for a string of suicide bombings they called revenge for military offensives in the northwest region, which borders Afghanistan. One attack killed nearly 70 people at a major weapons factory.

A military statement said residents of the Matta area, about 170km north of Peshawar, decided to fight back against militants operating in the area. They launched an attack on Friday night to prevent a suspected plan to snatch a village elder accused of supporting the government.

Six of the would-be kidnappers were killed. The militants later returned en masse, and ensuing clashes left 15 residents and three more militants dead, along with scores of people injured, the military said.

Country struggling with insurgents

Pakistan has struggled to contain rising militancy in its borders, and the fledgling government has tried both peace talks and military operations to stop the insurgents.

It's an effort watched closely by the US, which says militants have safe havens in Pakistan's northwest from which they plan attacks in neighbouring Afghanistan.

A recent US-led ground cross-border assault on a Pakistani tribal region, said to have killed at least 15 people, prompted protests from the government and angered many Pakistanis.

Asif Ali Zardari, widower of slain ex-prime minister Benazir Bhutto, won the presidency. He has vowed to be tough on militancy.