WASHINGTON (Reuters) - The United States is confident the top al Qaeda 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.

FILE PHOTO - Handout picture of Saudi fugitive Ibrahim Hassan al-Asiri as seen at the Saudi interior ministry of the most wanted terror suspects. REUTERS/Saudi Interior Ministry/Handout

Washington has long sought Ibrahim Hassan al-Asiri, a militant with al Qaeda’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.

“We are pretty confident that he has been killed,” one of the officials said.

That official added, however, that the usual fallout from a senior militant being killed, like a eulogy from Yemen’s al Qaeda branch, known as Al Qaeda in the Arabian Peninsula (AQAP), had not appeared. Separate U.S. intelligence officials said they do not consider the available evidence conclusive.

A report by U.N. experts monitoring Islamic State and al Qaeda for the U.N. Security Council, which was made public last week, said some states had reported that al-Asiri “may have been killed during the second half of 2017.”

“Given al-Asiri’s past role in plots against aviation, this would represent a serious blow to operational capability,” the U.N. experts wrote.

U.S. officials cautioned that while his death would be a major symbolic blow to AQAP, al-Asiri’s bomb-making skills almost certainly have been passed to others, and the threat from the group does not appear to have been significantly reduced.

The U.S. military and CIA have carried out strikes in Yemen. Neither commented on reports that al-Asiri had been killed.

AQAP has taken advantage of Yemen’s civil war to strengthen its position in the impoverished Arab state. The militant group still operates in several provinces in south and eastern Yemen.

Intelligence analysts believe that AQAP is one of the groups most capable of carrying out attacks against the United States.

Al-Asiri is believed to have masterminded the attempted Christmas Day, 2009, bombing of a U.S.-bound passenger jet. A Nigerian man is serving multiple life sentences in prison for trying to set off the bomb in his underwear.

The United States added al-Asiri to its terrorism blacklist in 2011 after he was believed to be the key suspect in the 2010 al Qaeda parcel bomb plot against the United States.

Al-Asiri was born in 1982 in Saudi Arabia to a military family and has been accused of recruiting his younger brother as a suicide bomber for a failed attack on former Saudi counter-terrorism chief Prince Mohammed bin Nayef in 2009.