BEIRUT (AFP) - Al-Qaeda has confirmed that top leader Abu Khayr al-Masri, believed to be the organisation's number two, was killed in an air strike by the US-led coalition in Syria.

In a statement, two branches of the militant group, including the powerful Al-Qaeda in the Arabian Peninsula (AQAP), called Masri a "hero" and said he 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 dated Wednesday (March 1) said, in reference to the US-led coalition bombing militants in Syria and Iraq.

It also expressed its condolences to Al-Qaeda's current leader, Ayman al-Zawahiri.

Masri is a son-in-law of Al-Qaeda founder Osama bin Laden and is believed to be Zawahiri's deputy.

A US official said this week that Washington was investigating reports that Masri had been killed in a US strike in or around the city of Idlib in northern Syria.

The Syrian Observatory for Human Rights monitoring group said earlier this week that a Feb 26 coalition raid on the town of Mastumah in Idlib province had killed Masri.

Egypt-born al-Masri, 59, was one of the most prominent figures in Al-Qaeda to have roots in the era before the Sept 11, 2001 attacks, according to the Soufan Group, a private security and intelligence consultancy.

"It was in al-Masri's guesthouse in Kabul, Afghanistan, that Khalid Sheikh Mohammed briefed top Al-Qaeda leaders about the planning of the Sept 11, 2001 attacks," the Soufan Group said.

His presence in Syria's northwestern Idlib province underscores the importance that country has gained in Al-Qaeda's strategy, analysts said.

Charles Lister of the Middle East Institute described al-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."