ISLAMABAD, Pakistan — The U.S. airstrike thought to have killed Taliban chief Akhtar Mohammad Mansour over the weekend represents another escalation in U.S. involvement in the war in Afghanistan and signals a new willingness to target senior Taliban leaders on Pakistani soil, analysts and officials said Sunday.

Although U.S. officials were awaiting final confirmation of Mansour's death, 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 believed to be the first time that the U.S. military directly targeted the top leader of the Afghan Taliban, which the U.S. government still does not officially designate a terrorist group.

"This is an unprecedented move to decapitate the Taliban leadership in its safe haven of Pakistan," said Bruce Riedel, a South Asia expert of 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 reaction on Sunday from Pakistani government and military leaders was fairly muted, even as Afghan officials cheered and described the attack as proof of the Afghan Taliban's deep presence in Pakistan.

Nafees Zakaria, a spokesman for the Pakistani Foreign Ministry, said that Pakistan has a "principled position that the Taliban must give up violence" and enter into peace talks with the Afghan government and that "military action is not the solution."

U.S. officials said the drone strike was justified because Mansour, who succeeded the late Taliban supreme commander Mohammad Omar last year, refused to negotiate with Afghan leaders and had been plotting to attack U.S. forces in Afghanistan.

Secretary of State John F. Kerry, who was traveling in Burma, said in a statement, "Mansour posed a continuing imminent threat to U.S. personnel in Afghanistan, to Afghan civilians, Afghan Security Forces, and Resolute Support coalition members across the country. And 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."

Kerry also said that Mansour was "directly opposed to peace negotiations and to the reconciliation process."

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 President Barack Obama, is believed to have been the first U.S. drone strike in that part of Pakistan, which includes the base of the Afghan Taliban insurgency.

Both Afghanistan's chief executive, Abdullah Abdullah, and the country's intelligence agency said Sunday that Mansour was killed in the strike. But U.S. military officials maintained that they did not have definitive proof of his death. In Pakistan, officials in Baluchistan said they recovered a charred vehicle and two bodies in the area.

The passenger, suspected of being Mansour, was using a Pakistani passport with the name Wali Mohammad registered to an address in Karachi. The driver 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 brother of the driver. "I don't know anything other than that."

In the aftermath of the strike, 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 14 ½-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.

Ahmad Saeedi, a former Afghan ambassador to Pakistan, said the attack sends "a clear message to terrorists that they cannot be safe in Pakistan."

Ghani has been pushing for such a message for several weeks.

After a truck bombing in Kabul about a month ago that killed at least 64 people, the Afghan president signaled that he may 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 that side of the border.

If Pakistan failed to act, he warned, Afghanistan would call for "responsible international entities" to "act outside of Afghanistan against the criminals whose hands are stained in the blood" of Afghans.

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 in Afghanistan 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.

Under their rules of engagement, however, 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.

But Brig. Gen. Charles H. Cleveland, chief spokesman for the U.S.-led coalition, said Obama authorized the strike because Mansour posed "a direct threat to U.S. forces."

"This presented an opportunity to eliminate the threat Mansour posed," Cleveland said.

According to U.S. officials, both Pakistani and Afghan leaders were notified after the strike occurred. Kerry personally called Pakistani Prime Minister Nawaz Sharif early Sunday, Pakistani media outlets reported.

Noting the limited reaction Sunday in Pakistan, some Pakistani analysts wondered whether Pakistan's military could have secretly sanctioned the airstrike. When Mansour was initially appointed, most analysts believed that Pakistan's military and intelligence had pushed for him to be named as Omar's replacement. But Mansour's resistance proved to be a major obstacle to peace talks, which Pakistan's military and government have supported.

Saad Muhammad, a retired Pakistani general who was Pakistan's defense attache to Kabul from 2003 to 2006, said he doubts Pakistan wanted Mansour killed. If his death is confirmed, Muhammad said, the Taliban could become even more splintered and peace talks become less likely.

"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."

After Omar's death was made public, several Taliban factions began fighting for control of the group. Hundreds of fighters were killed in internal clashes, but Mansour gradually appeared successful in uniting most strains of the insurgency behind him.

His death will touch off a new, perhaps even more complicated, leadership struggle.

Mansour infused 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. Coalition commanders have said that Sirajuddin Haqqani, who was named Mansour's top deputy, is taking a leading role in planning battlefield strategy.

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.

"The likely successor will be Sirajuddin Haqqani as, militarily, the Haqqani network has been more active militarily and politically recently in the Taliban movement," said Pir Mohammad Rohani, a former dean at Kabul University. "My short-term prediction is fighting may escalate."