MOGADISHU, Somalia -- Somalia's al-Qaida-linked rebels said Wednesday that they will kill the French intelligence agent they hold hostage.

The Islamist extremists said that France signed the death warrant of a French intelligence agent by launching a rescue operation last weekend that failed to free him, in a statement released Wednesday.

The militant group al-Shabab has held the French agent, Denis Allex, since July 2009. Al-Shabab said that it decided to kill Allex in retaliation for the late Friday to early Saturday operation. Two French soldiers and 17 Somalis were killed during the rescue attempt, French officials say.

French officials believe Allex is already dead. Al-Shabab has said Allex was still alive after the rescue attempt.

"It could have succeeded. It should have succeeded. Its consequences weigh heavily and I take full responsibility for the operation, because it is also a message from us: France cannot allow its citizens to be detained," French President Francois Hollande said at a press conference in Paris Wednesday.

Vague language from the Islamist extremists in Wednesday's statement does little to make things clear. Al-Shabab did not offer proof Allex is alive or say when he would be executed if he is still alive.

"With the rescue attempt, France has voluntarily signed Allex's death warrant," the statement said.

Adm. Edouard Guillaud, France's military chief of staff, said officials there believe al-Shabab's announcement is propaganda.

"We have had no indication since Friday night's raid that ... Denis Allex is alive," he told Europe 1. "We think that he is in all likelihood dead. ... It's a technique that they have already resorted to in other cases that didn't concern us."

Al-Shabab also said it had been willing to free Allex in exchange for "Muslim prisoners." It accused France of persecuting Muslims and pointed to a recently launched military operation by French forces against al-Qaida-linked extremists in Mali.

Al-Shabab claimed in its statement that Allex provided "a wealth of information" during interrogation sessions conducted over the last three years. They provided no proof for that claim.

Transported by helicopters, the French commandos attacked the al-Shabab position early Saturday in an attempt to free Allex. France's defence minister has said the government decided to stage the rescue a month ago, when Allex's location seemed to have settled down "in a spot accessible by the sea." U.S. military aircraft briefly entered Somali airspace to support the rescue operation, President Barack Obama said Sunday, but did not use weapons.

Al-Shabab provided their version of last weekend's attack. It said the French forces landed a few miles (kilometres) from the village of Bulomarer and walked to the targeted location, but that Islamist fighters knew they were coming and intercepted them. Al-Shabab said it had killed several spies in possession of "electronic gadgets" recently.

Al-Shabab said a battle raged for nearly three hours, and that attack helicopters came to the French fighters' assistance. Al-Shabab described their counterattack as having turned the French battle objective from a rescue mission to damage control.

French officials said they killed 17 of the Islamist rebels. Al-Shabab said many villagers were killed.

Al-Shabab once controlled all of south-central Somali, including the capital, Mogadishu. African Union troops pushed al-Shabab out of the capital in 2011, but the Islamist rebels still control wide swaths of rural southern Somalia.