Somalia's most dangerous militant group says it has killed a French hostage that French military forces tried to rescue last weekend.

Al-Shabab said in a Twitter posting on Thursday that the agent, codenamed Denis Allex, was executed on Wednesday evening Somalia time.

French officials have said they believe Allex was killed on the night of the raid. Allex, a French intelligence agent, was taken by the militants in July 2009 and had been held ever since.

A French raid on Friday night and early Saturday failed to rescue him, and resulted in the deaths of two French troops and 17 Somalis, French officials say.