Pakistan kept millions of pupils at home on Wednesday as it shut all schools and colleges after a suicide attack at a university, as the army became bogged down in an assault against the Taliban.

As ground troops pressed their offensive in a remote tribal region, police in cities went on heightened alert against yet more attacks from al-Qaeda and Taliban extremists in which 185 people have died so far this month.

Addressing his cabinet, Prime Minister Yousuf Raza Gilani said "these cowardly acts of terrorists will not go unpunished" and the government was "even more resolute in our commitment to eradicate the evil of militancy".

Some schools shut earlier this week and the rest of the country followed suit after two suicide bombers on Tuesday blew themselves up at Islamabad's International Islamic University, killing five people.

"All educational institutions are shut across the country," said an education ministry spokesperson.

"The closure has been ordered because of security threats received in recent days... We will assess the situation and hopefully they will reopen on Monday," the spokesperson, Atiqur Rehman, told AFP.

Parents not reassured

Gilani's tough talk failed to reassure parents who felt Tuesday's attack showed the militants can attack anywhere.

"We are under constant threat. Our children's education is suffering. If they can attack an Islamic university, they can do it anywhere," said Muhammad Irfan, whose five-year-old had to stay away from one of Islamabad's top private schools.

"We don't know what to do. We can't sacrifice our children but neither can we keep them at home."

Nazia Nawaz, mother of a playgroup student in another elite school, said the future of the nation was at stake.

"I am worried about the future of my daughter and my own. I don't know when this fear will pass, I don't know what is happening to us," she said.

Educational institutions 'soft targets'

In the populous Punjab province, universities and medical and engineering institutions were also closed amid fears of an attack.

"The closure is over threats that terrorists hitting soft targets may attack educational institutions," said Afzal Javed, district education officer in the central city of Multan.

The federal board of education, which looks after more than 600 schools and colleges in the country, said it had postponed examinations and a new date would be announced once the situation is improved.

Tuesday's university bombing was the seventh major militant attack in just over a fortnight, and the first since the military launched what officials vowed would be a knock-out blow against the Taliban in South Waziristan.

It came after a raid on army headquarters left 23 people dead and five UN World Food Programme workers were killed when a suicide bomber dressed in military uniform detonated his explosives in their Islamabad office.

Attacks planned by al-Qaeda

Authorities believe that many of the bombings in Pakistan, as well as attacks in the West, are being planned by al-Qaeda and Taliban militants in the lawless tribal region of South Waziristan.

Officials say at least 113 militants and 16 soldiers had been killed since the military offensive began there on Saturday, and more than 120 000 civilians have now fled the war zone.

Difficult terrain, mines and fierce clashes have slowed Pakistan's advance and officials admit the offensive could take longer than expected.

"The offensive may take more time than estimated. The area ? rough and difficult, mountain terrain is one of the reasons. Troops are advancing slowly, with a strategy and consolidating its positions," said one official.

There have been heavy clashes on the heights overlooking Kotkai, the home town of Pakistani Taliban chief Hakimullah Mehsud.

Numerous previous offensives in the tribal belt have had limited success, costing the lives of 2000 troops and ending generally with peace agreements that critics say gave the insurgents a chance to re-arm.