Somali militants linked to al-Qaeda say they have executed a French agent who armed forces had tried to rescue at the weekend.

The al-Shabaab rebel group said on its Twitter account that Denis Allex, held hostage since 2009, had been killed.

"Let Muslims enjoy his execution and the French cry," Sheikh Abdiasis Abu Musab, al-Shabaab's spokesman for military operations, told Reuters by telephone.

The militants said the decision to kill Mr Allex was unanimous and followed three years of what they called exhaustive attempts at negotiation.

France has consistently said it believed Mr Allex was killed in Saturday's failed rescue mission and have accused the militants of manipulating the media with their latest statements.

Two French commandos were killed in the raid, which coincided with the launch of French air strikes against Islamist militants in the west African country of Mali.