MOGADISHU, May 2 (Reuters) - Somalia's Islamist rebels vowed to fight on under new leadership on Friday after U.S. warplanes killed an insurgent said to be al Qaeda's commander in the Horn of Africa country.

Aden Hashi Ayro, who led al Shabaab militants blamed for attacks on government troops and their Ethiopian allies, was killed on Thursday in the latest of a string of U.S. air strikes on insurgents in the last year.

Security and intelligence sources say Ayro, in hiding since a U.S. air strike in January 2007, trained in Afghanistan in the late 1990s. He was one of six members or associates of al Qaeda thought by the United States to be in Somalia.

The Western-backed Somali government is trying to stem a rebellion that has been gaining ground but the rebels said the death of Ayro would not deter them.

"Even if Ayro has been martyred, his beliefs live on. The men who he trained and consulted are still around," Shabaab spokesman Mukhtar Ali Robow told local broadcaster Shabelle.

"We are warning the enemies of God that we will stay on the

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