The United States believes that a US strike in Syria has killed Mohsin al-Fadhli, the leader of a group of al-Qaida militants known as Khorasan, a US official told Reuters on Wednesday. 'We believe he is dead,' the official said, speaking on condition of anonymity. US officials have said the al Qaeda-linked group posed an immediate threat to the West and had been nearing the execution of a plot against US or European targets.