* France says believes agent already dead
* Militants say tried to negotiate his release for 3 years
* Militants want to impose Islamic law across Somalia
By Feisal Omar
MOGADISHU, Jan 16 (Reuters) - Somali militants linked to al Qaeda said on Wednesday they had sentenced a French agent to death after a failed attempt by French armed forces to rescue him at the weekend.
It was unclear whether the rebels were saying they had already killed Denis Allex, held hostage in Somalia since 2009. France said it had heard nothing since the military raid to alter its belief that he was killed during the rescue operation.
Al Shabaab said in a statement their decision to kill Allex was unanimous and followed three years of what it called "exhaustive attempts at negotiations" over his release.
The militants put up fierce resistance when French commandos flew into southern Somalia by helicopter under cover of darkness early on Saturday to try to free Allex.
Two of the commandos died in the raid.
"With the rescue attempt ... France has voluntarily signed Allex's death warrant," al Shabaab said in an emailed statement that was also posted on the group's official Twitter handle.
"It is the government of France ... which must bear full responsibility for the death of Allex," it said.
On Wednesday, Edouard Guillard, chief of staff for the French armed forces, told Europe 1 Radio there had been nothing since the raid to suggest Allex was alive and the rebels were engaged in "media manipulation."
"We think he is likely dead," Guillard said.
French President Francois Hollande, in a speech to the press later on Wednesday, said he took responsibility for the failed rescue operation of Allex, calling it "heavy with consequences."
"It involved the death, the assassination, of the hostage and two soldiers were taken," Hollande said. "I fully stand by this operation. Because it's also a message we're sending. France cannot accept that its nationals be taken."
Allex was one of two officers from the DGSE intelligence agency kidnapped by al Shabaab in Mogadishu in July 2009. His colleague, Marc Aubriere, escaped a month later but Allex had been held ever since in what Hollande on Wednesday called "abominable conditions".
In October, the militants uploaded a video of Allex pleading with Hollande to negotiate his release and save his life. Hollande said at the time the government was seeking to start talks with any party to facilitate his release.
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.
"Efforts were repeatedly hampered as the DGSE proved to be unreasonably apathetic and wilfully uncooperative," the rebels said.
Al Shabaab wants to impose their strict version of sharia, or Islamic law, across the Horn of Africa state, though it has lost significant territory in southern and central Somalia in the face of an offensive by African troops.
The rebel group, which formally merged with al Qaeda in February last year, is known to mete out beheadings and amputations and has banned music and football in areas under its control.