A U.S. hostage held by the Al-Qaeda group has been killed during a rescue operation in Yemen's southern Shabwah province, a senior Yemeni security source said on Saturday."Luke Somers was killed during a military operation to rescue him in Nisab town," the source, requesting anonymity, told The Anadolu Agency.The source, however, declined to give further details.Earlier, a military source had told the Yemeni Defense Ministry's website that Somers had been freed during the operation.His sister, however, told The Associated Press that her brother had been killed during a failed attempt to free him.Somers, 33, was abducted by the Al-Qaeda group in September 2013 in the Yemeni capital Sanaa.On Thursday, the Al-Qaeda group had threatened to kill Somers within three days unless Washington met its demands, which the group did not outline.The group said in a video on Thursday that it would kill Somers, 33, within three days if the U.S. failed to give into its demands.The group released a video of the U.S. hostage, in which urged U.S. authorities to intervene to rescue him.The Yemeni government says Al-Qaeda remains active in several parts of the country's south, from which it stages sporadic attacks on army and police personnel.Yemen has been dogged by turmoil since pro-democracy protests forced president Ali Abdullah Saleh to step down in 2012 after 33 years in office.