Afghan Taliban leader Mullah Akhtar Mansour has probably been killed in a US air strike, US officials say.
A drone targeted his vehicle in a remote area of Pakistan near the Afghan border, the officials said, adding that results were still being assessed.
US Secretary of State John Kerry said Mansour has posed "a continuing, imminent threat to US personnel".
Mansour assumed the leadership in July 2015, replacing Taliban founder and spiritual head Mullah Mohammad Omar.
The operation took place near the town of Ahmad Wal in Balochistan, south-west Pakistan, at around 15:00 (10:00 GMT) on Saturday and was authorised by President Barack Obama.
Both Pakistan and Afghanistan were informed about the strike shortly after it took place, the White House said.
Mr Kerry, on a visit to Myanmar, said: "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.
"Peace is what we want. Mansour was a threat to that effort."
A spokesman for Afghan President Ashraf Ghani said the drone strike "appeared to be successful", adding: "Our hope in the wake of the strike is for the Afghan-led peace process to bring lasting peace and stability."
Afghan Chief Executive Abdullah Abdullah said Mansour was "more than likely" dead, adding that he was "the main figure preventing the Taliban joining the peace process".
Analysis: Inayatulhaq Yasini, BBC Pashto online
The death of Mullah Akhtar Mansour, if confirmed, would be a big blow for the Taliban.
He was gradually tightening his grip on the movement by bringing into his fold other leading Taliban members, including a son and a brother of his predecessor Mullah Mohammed Omar, and by launching large scale attacks on Afghan security forces.
Under his leadership, the Taliban managed to capture an important city last year for the first time in 15 years.
Mansour also managed to silence the splinter Taliban group under Mullah Muhammad Rasool, which challenged his leadership, and is credited by his followers for containing so-called Islamic State in Taliban areas.
A vacuum created by his death would once again trigger a leadership struggle.
There have been conflicting reports from the Taliban.
Senior commander Mullah Abdul Rauf told Associated Press that Mansour had been killed, but that it was late on Friday.
Other reports denied his death. One unnamed Taliban commander told Reuters: "We heard about these baseless reports, but this not first time. Just wanted to share with you my own information that Mullah Mansour has not been killed."
False rumours have often surrounded Taliban leaders.
Omar died in 2013 but this was only confirmed by the Taliban two years later, while Mansour was reported to have been killed in a gun battle last year, something dismissed by the Afghan government.
Mansour's appointment as Taliban chief was disputed, with a rival group selecting their own leader.
Who is Mullah Mansour?
- Long seen as acting head of the Taliban, and close to its founder Mullah Omar
- Born in the 1960s, in Kandahar province, where he later served as shadow governor after the Taliban's fall
- Was civil aviation minister during the Taliban's rule in Afghanistan
- Had an active role in drug trafficking, according to the UN
- Has clashed with Abdul Qayum Zakir, a senior military commander, amid a power struggle and differences over negotiations with the Afghan government
- A man claiming to be Mansour met former Afghan President Hamid Karzai for peace talks in 2010 - but it later emerged he was an imposter
The Taliban have made gains since international troops withdrew from an active fighting role in 2014.
Nato forces are increasingly being deployed in battle zones to support Afghan forces fighting the Taliban.