Fierce fighting has broken out as Pakistan's army launched an air and ground offensive against Taliban militants in the South Waziristan area.
Officials said 30,000 troops, backed by artillery, had moved into the region where Pakistan Taliban leader Hakimullah Mehsud is based.
Militants were reported to be offering stiff resistance as troops advanced from the north, east, and west.
A curfew was imposed in the region before the offensive began.
There have been several co-ordinated Taliban attacks in recent days, killing more than 150 people in cities across Pakistan.
Pakistan's top army spokesman Maj Gen Athar Abbas confirmed that a fully-fledged assault had begun and said that an offensive could last up to two months.
"The objective is to clear this terrorist organisation from the area, who has taken over the area, turned these state institutions, organisations out and has taken the entire population hostage," he told the BBC.
He added that intense fighting was expected during the course of the operation.
Dozens of casualties have already been reported by local officials as both sides used heavy weapons.
The bodies of three Pakistan soldiers were taken to the northern town of Razmak. There have also been unconfirmed reports of militant deaths.
Nearly all communications in the region were down after the Taliban destroyed a telecommunications tower at Tiarza, local officials said.
Reports from the area are sketchy as it is difficult and dangerous for foreign or Pakistani journalists to operate inside South Waziristan.
Air attack
Aerial bombardments in the the Makeen area, a stronghold of the Mehsud tribe and a key army target, were also reported by local officials and witnesses.
One resident of Makeen town described the onset of fighting.
"We heard the sounds of planes and helicopters early Saturday. Then we heard blasts. We are also hearing gunshots and it seems the army is exchanging fire with Taliban," Ajmal Khan told the Associated Press news agency by telephone.
The ground operation comes after weeks of air and artillery strikes against militant targets in the region, which lies close to the Afghan border.
Thousands of civilians have fled South Waziristan in anticipation of the offensive.
Aid agencies say that many more are expected to flee but the tough terrain and the Taliban's grip on the area will present difficulties.
Transport has been difficult as roads have been blocked by the military.
There is a huge army presence on the road between Tank and Dera Ismail Khan, says the BBC's Islamabad correspondent Shoaib Hasan, near South Waziristan.
On his way to South Waziristan, he passed several army convoys on the road.
There has been no comment from the Pakistan military yet.
The mobilisation came a day after Pakistan Prime Minister Yusuf Raza Gilani held a meeting of the country's senior political and military leadership.
Lengthy planning
Recent militant attacks were seen as an attempt to divide public opinion, but they appear to have strengthened the resolve of the government, which says the Taliban must now be eliminated, our correspondent added.
The army has been massing troops near the militants' stronghold for months - ever since the governor of Pakistan's North West Frontier Province announced a ground offensive in South Waziristan on 15 June.
Pakistan's government has been under considerable pressure from the US to tackle militancy there.
North and South Waziristan form a lethal militant belt from where insurgents have launched attacks across north-west Pakistan as well as into parts of eastern Afghanistan.
South Waziristan is considered to be the first significant sanctuary for Islamic militants outside Afghanistan since 9/11.
