Pakistani troops backed by fighter jets launched a major operation against the Taliban in South Waziristan yesterday, sparking deadly clashes with heavily-armed rebels, officials said.

The mountain district is part of a tribal belt on the Afghan border that US officials call the most dangerous place in the world and is home to thousands of Taliban and Al-Qaeda-linked fighters branded a major threat to the West.

Air strikes and heavy artillery pounded Taliban bases as troops advanced north, west and east after months spent planning an assault that is expected to pose a stern test for the military on terrain ideally suited to guerrillas.

"The army has launched an operation after receiving orders from the government. The operation was launched early in the morning. Both air and ground troops are taking part," military spokesman Major General Athar Abbas told AFP.

Pakistan vowed to root out militants it branded a threat to the country's sovereignty after a spike in a two-year Taliban and Al-Qaeda-linked campaign of attacks that has made the country a frontline in the US-led war on terror.

Around 90,000 civilians have fled South Waziristan, normally home to 600,000 people, since August and officials said the number could more than double.

"More people are coming out," said Colonel Waseem Shahid, from an army support group. "We estimate that around 100,000 more people can be displaced, say a total of around 150,000 to 200,000 people can be displaced," he said.

Long-distance artillery pounded rebel positions as the vanguard of troops engaged in heavy clashes in mountainous forest at Sharwangi and Spinkai Raghzai, a local administration official told AFP on condition of anonymity.

Four soldiers were killed during clashes, the military said. Troops thrust north towards the Mehsud stronghold of Makeen, east towards Kotkai and Spinkai Raghzai, and west towards Kunigaram, Saam and Baddar, officials said. "They are using heavy weapons" against troops, one official told AFP. Others confirmed that troops had met resistance.

Pakistan has mounted numerous offensives against militants in its northwest since 2002, meeting with limited success and costing the lives of 2,000 troops.

"The operation will continue until the objectives are achieved. The army has blocked all entry and exit points of Waziristan," Abbas was quoted as saying by television channels that outlined an operation of six to eight weeks.

Military officials said the offensive would be swift, with the goal of finishing before the onset of harsh winter snows.