The US conducted a drone strike on Saturday against the leader of Afghan Taliban, likely killing him on the Pakistan side of the remote border region with Afghanistan in a mission authorised by US President Barack Obama, officials said.

The death of Mullah Akhtar Mansour, should it be confirmed, could further fracture the Taliban - an outcome that experts cautioned might make the insurgents even less likely to participate in long-stalled peace efforts.

The mission, which included multiple drones, demonstrated a clear willingness by Obama to go after the Afghan Taliban leadership in Pakistan now that the insurgents control or contest more territory in Afghanistan than at any time since being ousted by a US-led intervention in 2001.

Pentagon spokesman Peter Cook confirmed an air strike targeting Mansour in the Afghanistan-Pakistan border region but declined to speculate on his fate, although US officials speaking on condition of anonymity, told Reuters he likely was killed.

"We are still assessing the results of the strike and will provide more information as it becomes available," Cook said.

A Taliban commander close to Mansour, speaking to Reuters on condition of anonymity, denied Mansour was dead.

"We heard about these baseless reports but this not first time," the commander said. "Just wanted to share with you my own information that Mullah Mansour has not been killed."

In December, Mansour was reportedly wounded and possibly killed in a shootout at the house of another Taliban leader near Quetta in Pakistan.

Bruce Riedel, an Afghanistan expert at the Brookings Institution think-tank, described the US operation in Pakistan as an unprecedented move but cautioned about possible fallout with Pakistan, where Taliban leadership has long been said to have safe haven.

Husain Haqqani, a former Pakistani ambassador to the United States, predicted strained ties between the US and Pakistani militaries and said it would put Pakistan's powerful Inter-Services Intelligence (ISI) spy agency on alert.

"It is also a signal to the ISI that the US is losing patience with promises of Pakistan facilitating talks with the Taliban and is finally willing to strike at the Afghan Taliban leadership in Pakistan," he said.

A State Department official said both Pakistan and Afghanistan were notified of the strike but did not disclose whether that notification was prior to it being carried out.

"The opportunity to conduct this operation to eliminate the threat that Mansour posed was a distinctive one and we acted on it," the official said.

There was no comment immediately available from Afghan security and intelligence officials. The Pakistani military also did not respond to requests for comment.

TROUBLED PEACE TALKS

The US drones targeted Mansour and another combatant as the men rode in a vehicle in a remote area southwest of the town of Ahmad Wal, another US official said, speaking on condition of anonymity.

US special operations forces operated the drones in a mission authorized by Obama that took place at about 6 a.m. EDT (1000 GMT), the official said. That would have placed it at Saturday at 3 p.m. in Pakistan.

Cook called Mansour "an obstacle to peace and reconciliation between the government of Afghanistan and the Taliban" and said he was involved in planning attacks that threatened US, Afghan and allied forces.

Michael Kugelman, a senior associate for South and Southeast Asia at the Woodrow Wilson Center, said the most important target for the United States remained the top leadership of the Haqqani network, which is allied with the Taliban.

Mansour had failed to win over rival factions within the Taliban after formally assuming the helm last year after the Taliban admitted the group's founding leader, Mullah Omar, had been dead for more than two years.

It was unclear who Mansour's successor might be and Riedel said his death could create a crisis for the Taliban.

A US intelligence analyst said Mansour had been in a power struggle with Mullah Mohammad Rasoul, whose deputy, Mullah Dadullah, was killed late last year in what officials think was a fight with Mansour's more hard-line faction.

But the US official cautioned against concluding that the shakeup might diminish the Taliban's broader sense of strength, given political tensions in Kabul and the uneven performance of US-backed Afghan forces in recent months.

"The Taliban have made considerable progress in Helmand (province) and elsewhere so it's hard to see much incentive for them to start compromising now, with the fighting just heating up again," the official said.

The new US commander in Afghanistan is currently reviewing US strategy, including whether broader powers are needed to target insurgents and whether to proceed with plans to reduce the number of US forces. – Reuters