A Taliban faction leader who criticized the militant group's Pakistani head over attacks that killed civilians was fatally shot Tuesday, reportedly by one of his own guards.

The attack on Qari Zainuddin appeared to be a sign that divisions within the Taliban have broken into the open as they come under military assault. The army is clearing out militants from the Swat Valley and has been pounding strongholds of Pakistani Taliban leader Baitullah Mehsud in the South Waziristan tribal region bordering Afghanistan in apparent preparation for a major offensive.

Zainuddin was gunned down in nearby town of Dera Ismail Khan. He had emerged as Mehsud's chief rival.

Dr. Mahmood Khan Bitani told The Associated Press that he pronounced Zainuddin dead on arrival at a local hospital with gunshot wounds to the head and chest.

Baz Mohammad, an aide of the militant leader who also was wounded, said a guard barged into a room at Zainuddin's compound after morning prayers and opened fire. He accused Mehsud of being behind the attack.

"It was definitely Baitullah's man who infiltrated our ranks, and he has done his job," Mohammad told AP, vowing to avenge the death.

A spokesman for Mehsud could not immediately be reached for comment on the accusation.

Bahawal Khan, the area police official, confirmed the slaying, as did Sher Mohammad, an uncle of Zainuddin. Aides said the guard had gotten closer to Zainuddin about four months ago. He fled after the attack in a waiting car, they said.

Mahmood Shah, a former top security official, said the slaying sends a strong message to the government that they need to launch a strong, comprehensive operation to eliminate Mehsud, described as the center of gravity for much of the terrorist activity in Pakistan. Instead, Shah said, they have relied on "local efforts" by Mehsud's opponents like Zainuddin.

"Baitullah Mehsud has overcome all tribal dynamics. He has resources, funding and a fighting force to strike anywhere in Pakistan," Shah said, calling him a front man for al-Qaida. "You simply can't eliminate him through local efforts; instead, you need a major force."

Zainuddin was estimated to have about 3,000 armed followers in Dera Ismail Khan and nearby Tank. Earlier this month, he denounced Mehsud for recent attacks that have killed civilians — apparently launched in retaliation for the army offensive in the northwestern Swat Valley.

"Whatever Baitullah Mehsud and his associates are doing in the name of Islam is not a jihad, and in fact it is rioting and terrorism," Zainuddin told AP after a mosque suicide bombing attack, blamed on Mehsud, killed 33 people. "Islam stands for peace, not for terrorism."

Zainuddin's motive for criticizing Mehsud was not clear, but there was speculation that he was trying to portray himself as a more moderate alternative to the Taliban leader, although there appeared to be little or no differences between the two over fighting U.S. and NATO forces in Afghanistan.

Militants used mortars, rockets and an anti-aircraft gun to attack military positions in the northwest on Monday and were pummeled in response by airstrikes that killed at least 25 people, officials said.

It was the latest violence to break out in the tribal region on the Afghan border ahead of the expected offensive against Mehsud.

Army spokesman Maj. Gen. Athar Abbas on Monday offered the most detailed information yet about the military's goals for the operation in South Waziristan, which is also a potential hiding place of al-Qaida and Afghan Taliban leaders.

"Our effort is to break his network, the classes and training schools for suicide bombers running there," Abbas said of Mehsud. "To dismantle that ... and particularly the foreigners, who are in big numbers with him."

The government announced last week that the military would go after Mehsud in his stronghold in the remote mountainous region, where heavily armed tribesmen hold sway. The military also has been encouraging tribal leaders and other Taliban factions to rebel against Mehsud.

The operation comes on the heels of the military's offensive against the Taliban in Swat, which is now winding down.

Washington supports anti-militant operations, seeing them as a measure of nuclear-armed Pakistan's resolve in taking on a growing insurgency. The battle in the tribal region could also help the war in Afghanistan because the area has been used by militants to launch cross-border attacks on coalition troops there.

Daily bombing runs and artillery barrages have been softening up militant targets for about a week, and Abbas said the "pre-positioning" of ground troops in South Waziristan has been completed, though the campaign proper has not started.

Qari Hussain, a close aide of Mehsud, telephoned The Associated Press on Monday to say the military strikes had not weakened the Taliban in South Waziristan and had hit civilians and destroyed their homes. The military has been trying to avoid civilian casualties that could erode public support for the operation.