KABUL/WASHINGTON (Reuters) - The Afghan Taliban’s leadership council met on Sunday to start considering succession after a U.S. drone strike in Pakistan targeted its commander, two Taliban sources told Reuters, in the strongest sign yet the insurgency had accepted he was dead.

The strike targeting Mullah Akhtar Mansour on Saturday was perhaps the most high-profile U.S. incursion into Pakistan since the 2011 raid to kill al Qaeda leader Osama bin Laden and sparked a protest by Islamabad that its sovereignty had been violated.

If confirmed, Mansour’s death could trigger a succession battle within a Taliban insurgency that has proven extremely resilient despite a decade and a half of U.S. military deployments to Afghanistan.

The Taliban have not yet officially confirmed that Mansour was killed and there were conflicting accounts on Sunday, with the Afghan government declaring him dead, while Washington stopped just short of doing so.

“At this point, we’re not quite prepared to confirm that he was killed, though it appears likely,” U.S. Homeland Security Secretary Jeh Johnson told “Fox News Sunday.”

The Taliban sources said that Sunday’s meeting of the Rahbari Shura, or leadership council, included discussion of possible successors, including guerrilla commander Sirajuddin Haqqani.

Haqqani, who has a $5 million U.S. bounty on his head, would likely prove an even more implacable foe of Afghan government forces and their U.S. allies.

He is widely seen by U.S. and Afghan officials as the most dangerous warlord in the Taliban insurgency, responsible for the most bloody attacks, including one last month in Kabul in which 64 people were killed.

“Based purely on matters of hierarchy, (Haqqani) would be the favourite to succeed Mansour,” said Michael Kugelman, a senior associate at the Woodrow Wilson Institute think tank.

The Taliban were also considering Mullah Mohammad Yaqoob, the son of Taliban founder Mullah Mohammad Omar, a potential unifier because of his father’s name. Former Guantanamo detainee Mullah Abdul Qayyum Zakir and Mullah Sherin were also cited, the sources said.

PAKISTANI PROTESTS

The Saturday drone strike, which U.S. officials said was authorised by President Barack Obama, showed the United States was prepared to go after the Taliban leadership in Pakistan, which the government in Kabul has repeatedly accused of sheltering the insurgents.

Pakistan protested on Sunday, saying the U.S. government did not inform Prime Minister Nawaz Sharif beforehand.

“This is a violation of Pakistan’s sovereignty,” Sharif told reporters in London, saying it was still unclear who was killed.

A U.S. official, speaking on condition of anonymity, acknowledged that Washington only notified Pakistan after the operation.

It was unclear how long Mansour might have been inside Pakistan before the U.S. strike. Pakistan’s Foreign Ministry disclosed that a passport found at the site, bearing a different name, carried a valid Iranian visa.

It added that the purported passport holder was believed to have returned to Pakistan from Iran on Saturday, the day of the drone strike targeting Mansour. Photos of the passport, bearing the name Wali Muhammad, seen by Reuters showed a passing resemblance to some of the old photos available of Mansour.

Mullah Akhtar Mohammad Mansour, Taliban militants' new leader, is seen in this undated handout photograph by the Taliban. Taliban Handout/Handout via Reuters

If it is confirmed that Mansour had travelled to Iran before his death, it would raise new questions about the Taliban’s use of neighbouring territories, including Iran.

The drone strike underscored the belief among U.S. commanders that the Taliban under Mansour’s leadership had grown increasingly close to militant groups like al Qaeda, posing a direct threat to U.S. security.

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

U.S. Secretary of State John Kerry said the United States had conducted a precision air strike that targeted Mansour “in a remote area of the Afghanistan-Pakistan border”.

Mansour posed a “continuing, imminent threat” to U.S. personnel and Afghans, Kerry told a news conference while on a visit to Myanmar.

“If people want to stand in the way of peace and continue to threaten and kill and blow people up, we have no recourse but to respond and I think we responded appropriately,” Kerry said.

PEACE TALKS

Efforts to broker talks between the Afghan government and the Taliban had already stalled after a suicide attack in Kabul last month that prompted President Ashraf Ghani to prioritize military operations over negotiations.

But Ghani’s office said on Sunday the removal of Mansour could open the door to talks and that Taliban members who wanted to end bloodshed should return from “alien soil” and join peace efforts.

U.S. experts were less optimistic, cautioning against the idea that the shake-up would diminish the Taliban’s broader sense of strength, particularly given the uneven performance of U.S.-backed Afghan forces last year and a pullback by Western troops.

“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,” a U.S. intelligence official said on condition of anonymity.

The new U.S. commander in Afghanistan is reviewing strategy, including whether to request broader powers to target Taliban insurgents and whether to seek a halt in plans to reduce the number of U.S. forces in Afghanistan.

One U.S. government analyst said the Taliban were likely encouraged by the upcoming U.S. presidential election.

“The Taliban, the Haqqanis, (Pakistani intelligence) and everyone else knows that ... the election this fall isn’t going to bring in someone who wants to send more American troops to retake territory the Afghans are losing,” the analyst said.