DERA ISMAIL KHAN: Pakistani troops and Taliban fighters were locked in fierce clashes Tuesday in a major offensive that officials said had killed more than 100 people and uprooted 110,000.

Pakistan faces its toughest military test yet against the rebels in waging the ground assault in South Waziristan, part of a lawless tribal belt where Al-Qaeda and Taliban networks are allegedly plotting attacks on the West.

At least 20 militants and four soldiers died in overnight fighting, officials said, as troops pushed their offensive for a fourth day in the region adjacent to Afghanistan.

US Defence Secretary Robert Gates said he was "encouraged" by the offensive, which Pakistan says will deliver a knockout blow to Islamists blamed for a wave of bomb attacks that have killed 2,250 people in more than two years.

Before the announcement of the latest casualties, the army said 78 militants had been killed and nine soldiers.

The exact death toll is impossible to verify with the area sealed off and communication lines down.

Speaking on condition of anonymity, a senior military official said that "fierce fighting continued overnight between security forces and rebels".

Troops backed by artillery, attack helicopters and fighter jets pounded Taliban bastions around Kotkai, the home town of Pakistani Taliban chief Hakimullah Mehsud.

"The troops have yet to enter Kotkai and it seems that they (militants) want to defend this stronghold at all costs," the official said.

"Our troops fought a pitched battle overnight to gain control of Kotkai," another security official confirmed. Troops have also consolidated their positions in the Nawazkot and Sherwangi areas, he added.

Hoping to sow division, the military has been air-dropping leaflets urging Mehsud's tribe to rise up and fight alongside the army.

"The aim of the army is to provide an opportunity to the Mehsud tribe to live in peace and tranquillity," said the flyers.

The United States has long pressed for military action against Taliban and Al-Qaeda-linked militants, following a prolonged assault in the northwest Swat Valley -- considered a less difficult fight than Waziristan. Related article: US encouraged by offensive

"I think that the terrorist attacks that have been launched inside Pakistan in recent days made clear the need to begin to deal with this problem," Gates said, while cautioning that it was too soon to predict the outcome.

Nearly 30,000 troops are involved in the three-pronged push, unleashed after a recent string of spectacular attacks including a 22-hour siege on the army headquarters. They are up against about 10,000 Tehreek-e-Taliban fighters.

Officials say at least 110,500 people have fled to neighbouring districts Tank and Dera Ismail Khan, on foot and stuffed into pick-up trucks weighed down with bedding and animals, to escape the air strikes and fighting.

Numerous previous offensives against militants in the tribal belt have had limited success, costing the lives of 2,000 troops and ending generally with peace agreements that critics say gave the insurgents a chance to re-arm. Related article: Factfile on South Waziristan

Some of those forced to flee the fighting said this time would be no different.

"There is no likelihood of this operation being a success. It's just for show," 45-year-old shepherd Sharaf Khan told AFP on reaching Dera Ismail Khan with his young family after a three-day trek.

"We have spent the last four or five years living in misery. As far as we are concerned, neither the army nor Taliban are any good."