* France says believes agent already dead
* Al Shabaab militants say tried to negotiate release for three years
* Militants want to impose sharia 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.
Al Shabaab said in a statement the decision to kill Denis Allex, held hostage in Somalia since 2009, was unanimous and followed three years of what it called "exhaustive attempts at negotiations" over his release.
"With the rescue attempt ... France has voluntarily signed Allex's death warrant," the militants said in an emailed statement that was also posted on the group's official Twitter handle.
The militants put up fierce resistance when French commandos flew into southern Somalia by helicopter under the cover of darkness early on Saturday to try to free Allex. Later that day, the Paris government said it believed their hostage had been killed.
The raid to free Allex coincided with the launch of French air strikes on al Qaeda-affiliated rebels in Mali in West Africa, although French Defence Minister Jean-Yves Le Drian said the two military operations were unconnected.
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.
"We think he is likely dead," Guillard said.
It was not immediately clear whether the rebels were saying they had already killed Allex.
"It is the government of France ... which must bear full responsibility for the death of Allex," al Shabaab said.
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 Paris called "inhumane conditions".
The militants in October uploaded a video of Allex pleading with French President Francois 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 willfully uncooperative," the rebels said.
Guillard said the militants were engaging in "media manipulation."
Two French commandos were killed in the raid to free Allex that coincided with the launch of French air strikes in Mali in West Africa. The French defense ministry has said the two military operations were unconnected.
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 beaheadings and amputations and has banned music and football in areas under its control.