Tariq Hayat, the secretary of law and order for Pakistan's Federally Administered Tribal Areas, of which South Waziristan is part, said that despite South Waziristan's rugged terrain he expected the operation to be successful.

"A lot of planning has gone into it," he told Al Jazeera from Peshawar.

"This offensive is against people who have chosen to take up arms against Pakistan, who have chosen to fight against Pakistani troops and target innocent Pakistani civilians ... however, innocent people have nothing to fear from this operation."

By the afternoon, five soldiers were reported killed and 12 injured in clashes with Taliban fighters. Eleven fighters were killed, the Associated Press news agency reported.

Civilians flee

Wolfgang Herbinger, the country director for the World Food Programme, based in Islamabad, the capital, said his organisation had readied itself for the exodus from the fighting.

"We have food stocks in nearby areas," he told Al Jazeera.

In depth

  Video: Civilians flee Pakistani army offensive
  Video: Security crisis in Pakistan
  Video: Pakistan army HQ attacked
  Profile: Pakistan Taliban
  Witness: Pakistan in crisis
  Riz Khan: The battle for the soul of Pakistan
  Blog: Unsung heroes in an unseen war

"As a United Nations organisation we have been preparing for quite some time. We have had to anticipate in different parts of the country, including South Waziristan, that people will be on the move."

For weeks the army has been using air and artillery attacks on Taliban strongholds in South Waziristan and a curfew was put in place on Saturday.

Wajid Shamsul Hasan, Pakistan's high commissioner to the UK, said the offensive is difficult because the attacks are sporadic and spread out.

"It's difficult to fight them everywhere," he told Al Jazeera.

"That's why we were carrying out a softening operation through air raids and bombings. That's what we have been doing for the last four or five weeks.

"Now we think we are in a position to strike militarily on the ground in the spots we have been firing and mark them [Taliban] for attack. So I think we will be able to achieve our objective."

Intelligence officials said that Pakistani troops were advancing from Zhob to the south, Razmak to the north and Jandola from the east.

People are reported to be fleeing from the Shakoi and Zangra areas, with many moving through North Waziristan where a makeshift camp has been set up at Mir Ali.

The US hopes that a Pakistani army operation in South Waziristan will help break much of the opposition network that threatens both Pakistan and American troops across the border in Afghanistan.

The Pakistani offensive had been much talked about. It comes after a series of bomb attacks across the country over the past two weeks that have killed more than 170 people.

Sustained attacks

In the latest attack, at least 11 people died in two explosions near a police office in the city of Peshawar.

Facts: South Waziristan

 The district in the Federally Administered Tribal Area (FATA) borders Afghanistan, North Waziristan, the North-West Frontier Province and Baluchistan

 It has a population of about 500,000 people, mostly tribal Pashtuns, a religiously conservative group that is known for being hostile to outside interference

 The Pakistani Taliban holds territory mainly in the west-central region of South Waziristan, on the northern border with North Waziristan, towards the eastern town of Jandola and on the border with the North-West Frontier Province

 The Pakistani Taliban's bastion is not on the South Waziristan-Afghan border

 The army has launched offensives in South Waziristan before, initially in 2004 when it suffered heavy losses before signing a peace pact

Imran Khan, Al Jazeera's correspondent in Islamabad, said: "South Waziristan is where Hakimullah Mehsud, the leader of the Pakistani Taliban, is based.

"And the police say that the carnage we have seen over the last 12 days is being planned from South Waziristan.

"So this is a crucial operation to decapitate the head of the senior Pakistani Taliban leadership ... But there is always the problem that if you squeeze the Taliban in one area, they pop up in another."

Khalid Rahman, the director of the Institute of Policy Studies in Islamabad, said that the cause of the violence in Pakistan was the US war on the Taliban in Afghanistan, and that the Afghan conflict needs to be addressed for long-term success.

"The whole conflict has extended to Waziristan and then to some of the settled areas of Pakistan," he said.

"It is certainly the Pakistani government's responsibility to take care of things within Pakistan."

He said that the recent attacks had created a "favourable environment" for the military's assault on South Waziristan, but warned: "That is not going to last. Because the cause is Afghanistan and that remains."