Taliban militants are putting up fierce resistance to the Pakistan army as it attempts to oust them from strongholds in the remote South Waziristan region, BBC reported.

Dozens of casualties have been reported after the first day of the attack - the biggest offensive for six years.

Unconfirmed reports say 26 militants and five soldiers have been killed as heavy artillery and air strikes pound Taliban hideouts for a second day.

Thousands of civilians have fled the area for safety.

Up to 20,000 people have entered camps just outside the area, with aid agencies warning that many more could be displaced by fighting.

The military, mobilising from three directions, is controlling entry and exit points in the region.

Reports from the area are sketchy as it is difficult and dangerous for foreign or Pakistani journalists to operate inside South Waziristan.

Soldiers captured a Taliban stronghold at Spinkai Raghzai on Saturday after the militants withdrew to nearby mountains.

"It is a flat area, so whenever they tried to put up resistance, the helicopter gunships fired at them so they decided to flee to the mountains," a senior government official told Reuters news agency.

Troops established a checkpoint en route to Kotkai, the home town of Pakistani Taliban chief Hakimullah Mehsud, AFP reported.

There had been several co-ordinated Taliban attacks in the run-up to the offensive, killing more than 150 people in cities across Pakistan.

Security is tight in towns and cities throughout Pakistan in case the Taliban carry out revenge attacks.

Nearly all communications in the region were down after the Taliban destroyed a telecommunications tower at Tiarza, local officials said.

Aerial bombardments in the Makeen area, a stronghold of the Mehsud tribe and a key army target, were also reported by local officials and witnesses.

The ground operation comes after weeks of air and artillery strikes against militant targets in the region, which lies close to the Afghan border.

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.

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.