The al-Qaeda linked group also maintained that the hostage, Denis Allex, who France says it believes was killed during the operation, was still alive.

The militants put up fierce resistance when French special forces went into southern Somalia by helicopter under the cover of darkness on Saturday to try to free Allex.

There was some confusion over the exact outcome of the mission, with the French government saying at one point that one commando had died and the other gone missing and later saying that both appeared to be dead.

"The second commando died from his bullet wounds. We shall display the bodies of the two Frenchmen," Sheikh Abdiasis Abu Musab, spokesman for al Shabaab's military operations, told Reuters by telephone.

Allex's fate would be decided later, he said.

Defence Minister Jean-Yves Le Drian told reporters on Monday it seemed that both commandos were killed in the raid and said France was braced for some kind of gory video message by al Shabaab.

"Everything leads us to think that the hostage was assassinated and the other disappeared soldier has been killed," he said.

"And we are also, unfortunately, led to believe that al Shabaab is preparing to organise a macabre and shameful display of events."

A ministry source said earlier the government believed both commandos were dead, but it did not have the bodies.

Allex was one of two officers from France's DGCE intelligence agency kidnapped by al Shabaab three-and-a-half years ago in the capital Mogadishu. His colleague Marc Aubriere escaped a month later but Allex had been held ever since in what Paris called "inhumane conditions".

A video of a gaunt-looking Allex pleading with Hollande to negotiate his release and save his life appeared on a website in October used by Islamist militant groups around the world. Reuters could not verify its authenticity.

"Allex is alive and healthy," Musab said.

After Allex's abduction, al Shabaab issued a series of demands including an end to French support for the Somali government and a withdrawal of the 17,600-strong African peacekeeping force propping up the U.N.-backed administration.

Under pressure from the African troops and Somali government forces, the rebels have lost many of their urban strongholds, including Mogadishu, though they still wield influence in rural areas across southern and central Somalia.

Al Shabaab wants to impose their strict version of sharia, or Islamic law, across the Horn of Africa state.

The raid to free Allex coincided with the launch of French air strikes on al Qaeda-affiliated rebels in Mali in West Africa. Le Drian said over the weekend that the two military operations were unconnected.

French President Francois Hollande said on Saturday the Somalia operation had failed despite the "sacrifice" of two soldiers and "no doubt the assassination of our hostage".

Earlier that day France's defence ministry said one of the two Frenchmen was missing in action, stoking speculation that the soldier had been captured alive.