Q+A: Why is Pakistan attacking militants in Waziristan?

(Reuters) - Pakistani soldiers are zeroing in on two major Taliban sanctuaries in South Waziristan as their army presses ahead with an offensive in the lawless tribal region on the Afghan border.

(For a graphic showing the whereabouts of fighting, see here)

Here are some questions and answers on South Waziristan:

WHY HAS THE ARMY LAUNCHED AN OFFENSIVE?

South Waziristan is the main stronghold of the Tehrik-e-Taliban, or Taliban Movement of Pakistan, an alliance of more than a dozen militant groups that is fighting the government and wants to impose hardline Islamist rule.

The Pakistani Taliban have been responsible for a wave of violence across the country since mid-2007, when the army crushed an Islamist movement linked with South Waziristan based at Islamabad's Red Mosque.

As well as numerous suicide bomb attacks against military, government and foreign targets, the Pakistani Taliban are accused of killing former prime minister Benazir Bhutto in late 2007.

Taliban leader Baitullah Mehsud was killed by a missile fired by a U.S. drone aircraft on August 5 and replaced by Hakimullah Mehsud, who has vowed revenge.

Pakistani Taliban fighters virtually took control of the Swat valley, northwest of Islamabad, beginning in 2007 and a push out of the valley toward Islamabad spread fear early this year. The army responded with an offensive that has largely cleared militants from the valley.

The army is now fighting militants into their bastion in the hope of rooting out the most potent domestic threat to the state.

WHO IS THERE?

The army says about 10,000 hardcore fighters are in South Waziristan, although some analysts estimate more. Most are members of the region's ethnic Pashtun tribes who have battled outsiders for centuries.

Also there are foreign militants, including about 1,000 Uzbeks, some al Qaeda Arabs and even a handful of militants from Western countries. Militant factions from other parts of Pakistan, in particular the south of Punjab province, are also based with the Taliban in South Waziristan.

Osama bin Laden was believed to be hiding somewhere along the Afghan-Pakistani border, although analysts doubt he would remain for long.

WHAT ARE THE RISKS?

The South Waziristan offensive is seen as army's toughest test since the militants turned on the state.

The army first launched a brief offensives there in 2004, suffering heavy casualties before striking a peace pact.

The army has seldom, if ever, ventured into much of the semi-autonomous region of arid mountains and dried-up creeks and will be taking on fighters who have had years to prepare defenses.

The Pashtun tribes in the region have long been hostile to outside intervention and many locals -- particularly those belonging to the Mehsud tribe -- support the Taliban,. so some analysts say there is a possibility that the army could get bogged down.

Another risk for the army is that militant factions in North Waziristan might come to the help of their South Waziristan comrades while cells of militants in towns and cities could try to divert attention with urban attacks.