Pakistani soldiers pushed into the hometown of the country's Taliban chief, destroying his house Wednesday during a major offensive on an insurgent sanctuary close to the Afghan border, authorities say.

The military advances came as intelligence officials said suspected U.S. missiles killed three alleged militants in a neighbouring region where the army has tried to convince other insurgent factions to stay neutral during the offensive in South Waziristan.

The five-day-old offensive is considered a critical test of the nuclear-armed country's campaign against Islamist extremists blamed for soaring attacks at home and on Western forces in neighbouring Afghanistan.

All educational institutions in Pakistan were closed Wednesday after two suicide bombings at an Islamic university in Islamabad on Tuesday killed four people and injured 18.

The military is advancing on three fronts in South Waziristan, where the fight for Kotkai town is key because Taliban chief Hakimullah Mehsud and a top deputy, Qari Hussain, hail from there.

It also lies on the way to the major militant base of Sararogha. An army statement Wednesday said forces were engaged in "intense encounters" in heights surrounding Kotkai and had secured an area east of it.

Two intelligence officials said troops had secured parts of the town and destroyed houses belonging to Mehsud and Hussain in controlled explosions.

The military reported three more soldiers were killed, bringing the army's death toll so far to 16, while 15 more militants were slain, bringing their overall death toll to 105.

It is nearly impossible to independently verify information coming from South Waziristan, because the army has closed off all roads to the region. Analysts say both sides have exaggerated successes and downplayed losses in the past.

Up to 150,000 civilians left the region in recent months after the army made clear it was planning an assault, and as many as 200,000 people may flee in the coming weeks, authorities said.

With files from The Associated Press