Footage purports to show the aftermath of a U.S. airstrike in Pakistan that is thought to have killed Taliban chief Akhtar Mohammad Mansour on Saturday, May 21. (Zahid Gishkori)

The U.S. drone strike that killed Taliban chief Akhtar ­Mo­hammad Mansour represents another escalation of U.S. involvement in the war in Afghanistan by trying to cripple an insurgent group that has for years found refuge on Pakistani soil.

The strike early Saturday marks the most aggressive U.S. military action in Pakistan since the 2011 raid that killed Osama bin Laden. It is also thought to be the first time that the U.S. military has directly targeted the top leader of the Afghan Taliban, a potentially destabilizing action that could leave the group violently lashing out as it seeks to find a new leader.

President Obama called Mansour’s death “an important milestone.”

“We have a high-profile leader who has been consistently part of operations and plans to potentially harm U.S. personnel and has been resistant to the kinds of peace talks and reconciliation that could ultimately bring an end to decades of war in Afghanistan,” he said during a visit to Vietnam.

While Obama denied that the attack represented a shift in the U.S. approach, analysts see it as an escalation.

This image purports to show the vehicle in which Akhtar Mohammad Mansour was traveling. (Abdul Salam Khan/AP)

“This is an unprecedented move to decapitate the Taliban leadership in its safe haven of Pakistan,” said Bruce Riedel, a South Asia expert at the Brookings Institution. “It exposes Pakistan’s role in promoting and protecting the Taliban, and will provoke a crisis in U.S.-Pakistan relations.”

But unlike the bin Laden raid, which prompted outrage in Pakistan, the reported strike on Mansour drew a fairly muted reaction Sunday from Pakistani government and military leaders, even as Afghan officials cheered and described the attack as proof of the Afghan Taliban’s deep presence in Pakistan.

“While further investigations are being carried out, Pakistan wishes to once again state that the drone attack was a violation of its sovereignty,” the country’s Foreign Ministry said in a statement, adding that it was unable to confirm whether Mansour was killed.

What the apparent blow means for the Taliban remains uncertain. When Mansour took over after the death of longtime leader Mohammad Omar was announced last year, he had already effectively been running the group for two years, said Wahid Mozhda, a former Taliban diplomat who is now a political analyst in Kabul.

Appointing a successor now may be more challenging, with Omar’s eldest son, Mohammad Yaqob, and top deputies Sirajuddin Haqqani and Moulavi Haibatullah Akhunzada likely to be vying for control.

“It is very hard to anticipate who would be Akhtar Mansour’s successor among these three men,” Mozhda said.

Pentagon officials say Taliban leader Mullah Akhtar Mohammad Mansour was likely killed in a drone strike in Pakistan on May 21. (Reuters)

If Mansour’s death is confirmed (some Taliban supporters deny he was killed), the strike will cause even more turmoil for an insurgency that was already showing signs of fraying, despite continued success on the battlefield.

Although Mansour, working through proxies, succeeded in quelling several insurrections against his leadership, he was also apparently a man on the run.

Mansour was not in direct contact with senior Taliban leaders, who received directives from him via emails issued by close aides or through audio tapes, said Rahimullah Yusufzai, a journalist and longtime Taliban observer from Peshawar. “He had adopted the policy of living life as a phantom.”

Pakistan’s Foreign Ministry has not confirmed Mansour’s death. A statement said officials recovered the body of a man named Wali Muhammad, believed to be Mansour’s alias. That man had been in Iran and only entered Pakistan on Saturday, the day of the drone strike.

“His passport was bearing a valid Iranian visa,” the statement said.

The body is believed to still be in a hospital in Quetta.

Despite Mansour’s possible absence, the Taliban has been carrying out daily operations throughout Afghanistan, some of which involved massing dozens of militants for days-long battles against Afghan security forces.

For Taliban leaders, a key question is whether Saturday’s drone strike will be followed up by additional U.S. military actions in southwestern Pakistan.

Saad Muhammad, a retired Pakistani general who was Pakistan’s defense attache to Kabul from 2003 to 2006, said the Taliban will face “a very difficult choice.”

“If they remain in Quetta, in their comfort zone, they will have to deal with some Pakistani pressure to leave,” he said. “But if they go out, they will have to deal with attacks that could be life-threatening.”

U.S. officials said the drone strike was justified because Mansour refused to negotiate with Afghan leaders and had been plotting to attack U.S. forces in Afghanistan.

“This action sends a clear message to the world that we will continue to stand with our Afghan partners as they work to build a more stable, united, secure, and prosperous Afghanistan,” Secretary of State John F. Kerry, who was traveling in Burma, said in a statement Sunday.

In a statement late Saturday, the Pentagon said several unmanned U.S. aircraft struck a vehicle in which Mansour was traveling in western Pakistan’s Baluchistan province. The strike, authorized by Obama, is thought to have been the first U.S. drone strike in that part of Pakistan, which includes the base of the Afghan Taliban insurgency.

Local officials in Baluchistan said they recovered a charred vehicle and two bodies.

The passenger, suspected of being Mansour, had a Pakistani passport registered to an address in Karachi. The other man was apparently a taxi driver, local officials said.

“On Thursday night, he told me that he’ll be on a long drive with a passenger coming from Afghanistan,” said Mohammad Qasim, the driver’s brother. “I don’t know anything other than that.”

Afghan President Ashraf Ghani was in Qatar for a two-day state summit. But Ghani’s spokesman described the strike as a potential turning point in the 14½ -year-old Afghanistan conflict.

“This shows the strong U.S. resolve in fighting those who are against peace and are terrorists,” said Dawa Khan Mina Pal, the spokesman.

After a truck bombing in Kabul about a month ago that killed at least 64 people, the Afghan president signaled that he might try to get the United States to expand the war into Pakistan. In a speech before parliament, Ghani said he had all but given up on the peace process and urged Pakistan to take decisive action against Taliban militants on its side of the border.

But with just 9,800 American troops on the ground, Obama has been trying for months to transition the U.S. military out of direct offensive action in Afghanistan. About 6,600 troops are based there as part of the NATO mission to train Afghan security forces.

The remaining U.S. troops are stationed there for counterterrorism missions targeting al-Qaeda and the Islamic State. The United States does not officially designate the Afghan Taliban as a terrorist group.

But under their rules of engagement, U.S. forces are allowed to take defensive action when threatened by the Taliban.

It was not clear how a kill strike against Mansour in Pakistan fits into the criteria. Most previous U.S. drone strikes in Pakistan were carried out by the CIA in the northwestern tribal belt.

Noting the limited reaction Sunday in Pakistan, some Pakistani analysts wondered whether Pakistan’s military could have secretly sanctioned the airstrike.

Muhammad, the retired Pakistani general, said he doubts Pakistan wanted Mansour killed.

“Obviously, they want a Taliban group that remains united because, if fragmented, it becomes much more difficult to control,” said Muhammad, who still maintains contact with some elements of the Taliban leadership. “This will create a very difficult situation for Pakistan, especially due to expectations Pakistan should bring them to the peace table.”

Mansour brought the leadership of the Haqqani network, a somewhat independent offshoot of the Taliban that the United States considers a terrorist group, into his command structure.

Some Afghan analysts believe that Haqqani, known for employing especially brutal tactics against coalition forces and foreigners, is now well positioned to assume full control over the Taliban.

But Mansour’s other two deputies, Akhundzada and Yaqob, are also considered top contenders for the job.

Both Muhammad, the retired Pakistani general, and Mozhda, who worked as a diplomat when the Taliban controlled Kabul before 2001, think Yaqob would have the upper hand in a leadership struggle.

Abdul Qayoum Zakir, a former Guantanamo Bay detainee and a member of the Taliban executive council, could be a dark-horse candidate, Muhammad said.

Regardless, the leadership struggle may not be accompanied by a lull in violence.

After Omar’s death was announced last summer, the Taliban launched three major attacks in Kabul within 24 hours, killing more than 50 people.

Olivo reported from Kabul, and Ryan reported from Washington. Aamir Iqbal in Peshawar, Pakistan; Shaiq Hussain and Zahid Gishkori in Islamabad; Greg Miller and Carol Morello in Washington; David Makamura in Hanoi; and Sayed Salahuddin and Mohammad Sharif in Kabul contributed to this report.