Al-Qaeda has confirmed that top leader Abu Khayr al-Masri, believed to be the organisation's number two, was killed in a drone strike by the US-led coalition in Syria.
The killing of Masri, described by analysts as "jihadi royalty," could serve as a major security coup for US President Donald Trump early in his presidency.
Two branches of the global jihadist group, including the powerful Al-Qaeda in the Arabian Peninsula (AQAP), announced Masri's death in a statement dated Wednesday.
Calling him a "hero," the statement said Masri "was killed during a Crusader drone strike" in Syria.
"All of al-Sham (Syria) will bear witness to the latest crime of America and the Crusader alliance," the statement said, in reference to the US-led coalition bombing jihadists in Syria and Iraq.
It also expressed its condolences to Al-Qaeda chief Ayman al-Zawahiri.
A US official said this week that Washington was investigating reports that Egyptian-born Masri had been killed in a US strike in or around the city of Idlib in northwest Syria.
The US has escalated its bombing campaign in the province, mostly ruled by Al-Qaeda's former Syrian branch, now known as Fateh al-Sham Front.
In early February, the Pentagon said it killed 11 Al-Qaeda operatives there.
Masri was a son-in-law of Qaeda founder Osama bin Laden and believed to be Zawahiri's deputy.
Also known as Abdullah Muhammad Rajab Abdulrahman, Masri joined Zawahiri in the Egyptian Islamic Jihad group in the 1980s before they enlisted with Bin Laden in the 1990s.
US intelligence believe Masri was involved in the 1998 bombings of US embassies in Kenya and Tanzania.
He was detained with several other Al-Qaeda figures in 2003 in Iran and held until 2015, when they were traded for the release of an Iranian diplomat who had been seized by Al-Qaeda's Yemen branch.
Charles Lister of the Middle East Institute described Masri as "jihadi royalty," as a longstanding member of Al-Qaeda's central Shura Council and "one of Ayman al-Zawahiri's closest long-time confidants."
The Qaeda statement warned the "US and its allies, agents and spies that the more martyrs they kill, the more determined we will be to achieve victory and avenge them."
Trump has put tackling so-called "radical Islamic extremism" at the top of his agenda.
He is considering a review of the fight against the Islamic State group, aiming to intensify the campaign, and is poised to revive efforts to ban travel from certain Muslim-majority countries.
Masri's presence in Idlib province underscores the importance that Syria has gained in Al-Qaeda's strategy, analysts said.
Masri was born in the Nile Delta city of Kafr el-Sheikh in November 1957, during Gamal Abdel Nasser's rule.
He was one of the most prominent figures in Al-Qaeda to have roots in the era before the September 11, 2001 attacks, according to the Soufan Group, a private security and intelligence consultancy.
"It was in Masri's guesthouse in Kabul, Afghanistan, that Khalid Sheikh Mohammed briefed top Al-Qaeda leaders about the planning of the September 11, 2001 attacks," the Soufan Group said.
According to Treasury Department sanctions, Masri was previously responsible for coordinating Al-Qaeda's work "with other terrorist organisations."