WASHINGTON - The drone airstrike that reportedly killed Taliban chief Mullah Mansour represents another escalation in US involvement in the war in Afghanistan and signals a new willingness to target senior Taliban leaders on Pakistani soil, American analysts and officials said Sunday.

They also said that Mullah Mansour’s death could add to the Taliban’s leadership difficulties less than a year after the death of their founding leader, Mullah Muhammad Omar, was revealed.

The strike early Saturday marks the most aggressive US military action in Pakistan since the 2011 raid that killed Osama bin Laden. It is also believed to be the first time that the US military directly targeted the top leader of the Afghan Taliban, which the US 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,” Bruce Riedel, a former CIA officer, told The Washington Post on Sunday. “It exposes Pakistan’s role in promoting and protecting the Taliban, and will provoke a crisis in US-Pakistan relations,” said Ridel.

US lawmakers welcomed the news and some urged the Obama administration to take a firmer stand. “I appreciate President Obama for authorising the attack”, Republican Senator Lindsey Graham said, adding “I strongly encourage the Obama administration to not withdraw troops until conditions on the ground permit their withdrawal”.

Saturday’s mission, which USA officials said was authorised by President Barack Obama and included multiple drones, showed the United States was prepared to go after the Taliban leadership in Pakistan.

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

US military officials are still maintaining that they did not have definitive proof of Mansour’s death.

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

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 reportedly been trying for months to transition the US 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 US troops are stationed there for counterterrorism missions targeting Al-Qaeda and the Islamic State.

Under their rules of engagement, however, US 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,” the Post said. Most previous US drone strikes in Pakistan were carried out by the CIA in the northwestern tribal belt.

But Brig Gen Charles Cleveland, chief spokesman for the US-led coalition, said Obama authorised the strike because Mansour posed “a direct threat to US forces.”

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

The Washington Post dispatch said, “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 defence attache to Kabul from 2003 to 2006, said he doubts Pakistan wanted Mansour killed. If his death is confirmed, he said, the Taliban could become even more splintered and peace talks become less likely.

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,” Pir Mohammad Rohani, a former dean at Kabul University, was quoted as saying. “My short-term prediction is fighting may escalate.”

According to a New York Times dispatch, Mullah Hameedi, a top Taliban military commander in southern Afghanistan who confirmed a strike in the border area but denied that Mullah Mansour was there, said: “We are going to persuade Mullah Mansour to publish his audio record to confirm he is alive.”