ISLAMABAD: Pakistan's capital Islamabad was on high alert Sunday after a deadly suicide blast killed two policemen, as clashes broke out between Taliban fighters and angry vigilantes in the northwest.

Also, Prime Minister Yousuf Raza Gilani has insisted that a string of recent bomb blasts widely seen as retribution for a six-week-long army offensive in the northwest would not deter their anti-Taliban push.

Two policemen were killed late Saturday when a suicide bomber walked up to a police emergency helpline centre in an Islamabad residential district home to many government officials and detonated explosives strapped to his body.

Witnesses said police shot the suicide bomber before he was able to enter the building but that he was nevertheless able to detonate the bomb.

The night-time assault came after another suicide blast killed 38 people during Friday prayers at a mosque in Upper Dir, near three northwest districts hit by the military offensive aimed at crushing Taliban fighters.

Police and local officials said up to 1,000 Upper Dir villagers formed a vigilante mob Saturday and stormed three villages where Taliban were hiding out, killing six suspected militants to avenge the mosque bombing.

Sporadic fighting continued in the area Sunday, as tensions rose in the nuclear-armed nation and police heightened security in the capital.

"Islamabad police are on high alert. We have taken tough security measures after the bombing on Saturday," a senior police official told AFP, asking not to be named as he was not authorised to speak to the media.

"They are conducting surprise checks of vehicles."

No group has yet claimed responsibility for Saturday night's suicide attack.

Since the northwest offensive began in late April more than a dozen bomb blasts have killed over 100 people, with Peshawar, the main city in the northwest, Pakistan's cultural capital Lahore and now Islamabad all hit.

Gilani condemned the Islamabad attack, calling it a "cowardly act of terrorism," a statement from his office said late Saturday.

"The prime minister reiterated his government's resolve to stamp out the menace of militancy and terrorism, adding that such incidents would not deter the government (from its) commitment to eliminate this scourge," it added.

Saturday's suicide blast was the first such attack in Islamabad in more than two months, after a suicide bomber killed eight paramilitary police at a camp on April 4.

Bomb attacks across Pakistan blamed on Islamist extremists have killed more than 1,900 people in the past two years.

The northwest offensive has broad public support, and the vigilante attack in Upper Dir signalled rising tensions between locals and the extremists bent on imposing a harsh brand of Islamic law.

Mohammad Ejaz Ahmed, Upper Dir police chief, said the vigilantes chased Taliban fighters out of three villages and torched 19 houses during clashes.

Local administration chief Atif-ur Rehman confirmed that six suspected insurgents had been killed, and said authorities had imposed a ban on gatherings of more than five people in the area.

Sporadic gunfire could still be heard in Shatkas village, he added.

Pakistan launched its northwest push after the Taliban advanced to within 100 kilometres (60 miles) of Islamabad in early April, violating a deal to put the region's three million people under sharia law in exchange for peace.

Operations are now focused on the Taliban bastion of Swat valley, where Pakistani jets and helicopter gunships Sunday pounded militant strongholds and troops hunted down fighters allegedly holed up in the rugged mountains.

An army statement Sunday said that four suspected militants and three soldiers had died in the past 24 hours. The military claim to have killed 1,300 militants in the offensive, although the death toll are impossible to verify.

Elsewhere in the troubled northwest, four people were killed in a clash between rival factions of the Pakistani Taliban in the tribal region of Bajaur near the Afghan border, local administration official Faramosh Khan said.