The Taliban's most prominent military commander, a one-legged fighter who orchestrated an ethnic massacre and a rash of beheadings, was killed in a U.S.-led military operation in southern Afghanistan, officials said Sunday.

Mullah Dadullah, a top lieutenant of Taliban leader Mullah Omar, was killed Saturday in the southern province of Helmand, said Said Ansari, the spokesman for Afghanistan's intelligence service. NATO confirmed his death, calling it "a serious blow" to the insurgency.

Dadullah is one of the highest-ranking Taliban leaders killed since the fall of the hard-line regime following the U.S.-led invasion in 2001. His death represents a major victory for the Afghan government and the international coalition that has struggled to contain a Taliban-led insurgency wracking the south and east of the country.

"Mullah Dadullah was the backbone of the Taliban," said Asadullah Khalid, governor of the former Taliban stronghold of Kandahar. "He was a brutal and cruel commander who killed and beheaded Afghan civilians."

Khalid showed Dadullah's body to reporters at a news conference in the governor's compound. An Associated Press reporter said the body, lying on a bed and dressed in a traditional Afghan robe, had no left leg and three bullet wounds: one to the back of the head and two to the stomach.

The reporter said the body appeared to be Dadullah's based on his appearance in TV interviews and Taliban propaganda videos.

But Qari Yousef Ahmadi, a purported Taliban spokesman, denied that the Taliban commander had been killed.

"Mullah Dadullah is alive," Ahmadi told AP by satellite phone. He did not give further details.

NATO's International Security Assistance Force confirmed the death, saying that after Dadullah left his "sanctuary" in the south, he was killed in a U.S.-led coalition operation supported by NATO and Afghan troops.

Dadullah "will most certainly be replaced in time, but the insurgency has received a serious blow," the ISAF statement said.

Rahimullah Yusufzai, a Peshawar-based editor for the Pakistani newspaper The News and an expert on the Taliban, said Dadullah's death would be a huge blow for the militant group.

"I think this is the biggest loss for the Taliban in the last six years," Yusufzai said. "I don't think they can find someone as daring and as important as Dadullah."

But Yusufzai and Mustafa Alani, director of security and terrorism studies at the Dubai-based Gulf Research Center, said the death would have little long-term effect. Alani noted that insurgent attacks in Iraq did not abate after the killing of Al Qaeda's leader in Iraq, Abu Musab al-Zarqawi, last June.

"In this sort of organization, people are replaceable, and always there is a second layer, third layer. They will graduate to the leadership," Alani said. "He is important, no doubt about it. Yes, it is a moral victory, but he's replaceable."

Yusufzai said many Taliban fighters had been unhappy with Dadullah, saying he maligned the militant group with brutal beheadings, a rash of kidnappings and boastful videos that starred himself shooting weapons and walking in Afghanistan's mountains.

"They thought he had become too big for his shoes," Yusufzai said.

No official would give an on-the-record account of Dadullah's death, and several gave conflicting reports, speaking only on condition of anonymity because of fear or because they were not allowed to be quoted.

A tribal leader from the Nad Ali district of Helmand province said ground forces and helicopters surrounded a home in Dadullah's home village of Kakeban, killing the Taliban leader and seven militants. A Helmand government official, meanwhile, said Dadullah was killed while traveling in his vehicle in southern Helmand.

Both officials said Dadullah was killed early Saturday morning, around 3 a.m.

An intelligence service official said Dadullah was killed near the Sangin and Nahri Sarraj districts of Helmand province, an area that has seen heavy fighting the last several weeks involving British and Afghan troops and U.S. special forces.

In December, a U.S. airstrike near the Pakistan border killed another top Taliban commander in southern Afghanistan, Mullah Akhtar Mohammad Osmani. Dadullah, Osmani and policy-maker Mullah Obaidullah had been considered to be Omar's top three leaders.

Dadullah, who comes from the southern province of Uruzgan, lost a leg fighting against the Soviet army that occupied Afghanistan in the 1980s. He emerged as a Taliban commander during its fight against the Northern Alliance in northern Afghanistan during the 1990s, helping the hard-line militia to capture the city of Mazar-e-Sharif.

An ethnic Pashtun, the group that makes up the core of the Taliban and is prominent in eastern and southern Afghanistan, Dadullah led a Taliban massacre of ethnic Hazaras in 1999 in the province of Bamiyan, where the Taliban in 2000 destroyed two large, ancient Buddha statues carved into a hillside cliff.

Since the Taliban's ouster in late 2001, Dadullah emerged as the group's most prominent and feared commander. He often appeared in videos and media interviews, and earlier this year predicted a militant spring offensive that has failed to materialize.

In an interview shown on Al-Jazeera on April 25, Dadullah claimed Al Qaeda leader Usama bin Laden was behind the February attack outside a U.S. military base in Afghanistan during a visit by Vice President Dick Cheney — although the U.S. military this month claimed a Libyan Al Qaeda operative, Abu Laith al-Libi, not bin Laden, was behind it.

The interview was not the first time in recent months that Dadullah has said bin Laden is alive. On March 1, London television Channel 4 aired an interview in which he said the al-Qaida leader was in contact with Taliban officers. The station did not say when the tape was made.

In separate fighting, Afghan and international forces killed 55 suspected Taliban in two operations near the border with Pakistan after officials learned of a plan to attack a local security post, said Ghamai Khan, spokesman for the governor of the province of Paktika. A policeman was also killed.

"The governor knew about the enemy operation, so they launched an operation and airstrike," Khan said.