The United States is confident the top al-Qaida bomb maker, believed to be the mastermind behind a failed bombing of a U.S.-bound airliner in 2009, has been killed, two U.S. officials said even as others cautioned the evidence was not conclusive.

Washington has long sought Ibrahim Hassan al-Asiri, a militant with al-Qaida's Yemen branch who is one of the world's most feared bomb makers because of his ability to create hard-to-detect bombs, including some implanted in suicide bombers.

Two U.S. officials, including a senior official, said they were confident al-Asiri had been killed. They were speaking on the condition of anonymity.