Pakistani forces launched a ground offensive against Taliban militants in South Waziristan on the Afghan border on Saturday, with soldiers advancing from three directions, officials said.
The assault follows a string of militant attacks that began on October 5 with the suicide bombing of a UN office in Islamabad and included assaults on the army headquarters, police and the public in which more than 150 people were killed.
In a show of unity before the offensive, government and political party leaders gave the military full backing on Friday, vowing to weed out militants and establish the writ of the state.
"The operation has started," said military spokesman Major General Athar Abbas said. He declined to give details or say how long he expected the offensive would take.
Intelligence and government officials said troops were moving from three directions and some clashes had erupted.
The militants were firing rocket-propelled grenades, machine guns and anti-aircraft guns while government forces were using artillery, mortars and aircraft, intelligence officials said. Four soldiers were wounded, an official said.
The army says about 28,000 soldiers are taking on an estimated 10,000 hard-core Taliban, including about 1,000 tough Uzbek fighters and some Arab al Qaeda members.
The army has for months been using aircraft and artillery to to soften up the militants' defences while moving in troops to seal off the region. The army had stepped up air and artillery attacks in recent days.
Army chief General Ashfaq Kayani briefed government and party leaders on Friday and they all agreed the militants posed a serious threat to the sovereignty and integrity of the state.
"The national consensus is reaffirmed to establish and maintain the writ of the state to weed out these elements," Prime Minister Yusuf Raza Gilani's office said.
Up to 100,000 people had fled from South Waziristan in anticipation of the offensive, Abbas said, while the United Nations said 500 people were leaving every day.
Many al Qaeda and Taliban members fled to northwest Pakistan after US-led troops ousted the Taliban in Kabul in 2001 and the region has become a global hub for Islamist militancy.
The offensive could be the army's toughest test since the militants turned on the state and it will be hoping Afghan Taliban factions elsewhere in South Waziristan and in North Waziristan stay out of the fight.
Fleeing residents said heavy exchanges of fire had erupted.
"There was heavy firing from both sides and people are fleeing," villager Alluddin Mehsud told Reuters after arriving in the safety of Dera Ismail Khan city, east of Waziristan.
Telephone services had apparently been cut to the area. Foreign reporters are not allowed in and many Pakistani reporters based there have left.
Earlier, a government official said authorities had imposed a curfew along roads in South Waziristan to protect forces moving towards militant strongholds.
Roadside bombs went off near military convoys in both the South and North Waziristan regions, killing three soldiers and wounding five, another intelligence official said.
Pakistani Taliban made advances towards Islamabad early this year, raising fears about the stability of the US ally.
But significant military gains in the Swat valley, northwest of Islamabad, have eased US concerns about Pakistan's commitment to the fight.
In a sign of US support, President Barack Obama signed on Wednesday a $7.5 billion aid bill for Pakistan for five years.
But Pakistan's military had complained about the bill because the legislation ties some funds to fighting militants and is seen by critics as violating sovereignty..
US officials in Washington said on Friday the Pentagon was ramping up delivery of military equipment long sought by the Pakistani army to fight militants.