BEIRUT (Reuters) - An air strike carried out by an unidentified warplane killed seven Hezbollah fighters in eastern Syria, three sources familiar with the incident told Reuters on Monday. The identity of the warplane was not confirmed, but the sources did not rule out the possibility that it was Russian  friendly fire . The sources did not say when the air strike happened. The air strike struck a Hezbollah position in eastern Homs province, where the Iran-backed Lebanese group is fighting Islamic State alongside the Syrian and Russian militaries. Asked about a report that a U.S. drone carried out the strike, the spokesman of the U.S.-led Coalition said the location was outside its area of operations. The coalition is also conducting air strikes in Syria against Islamic State in support of the Syrian Democratic Forces, an alliance of Arab and Kurdish militias. 