A loud explosion that occurred Sunday in the western Beqaa region of Lebanon Sunday, between the town of Sahbin and Jabal al-Barouk, is the subject of speculation. Al Jazeera TV ascribes it to a missile attack by an Israeli jet. Lebanese sources speak of an Israeli drone crash. Beirut reports earlier flights by an Israeli surveillance plane over Mount Hermon and Hasbaya. A Lebanese army unit has sealed off the site of the blast. DEBKAfile: The explosion may have been caused by Syrian rebel rocket fire against Hizballah strongholds. Al Qaeda-linked Nusra Front forces have infiltrated Lebanon in recent days and reached a point not far from Baalbek, the main town of the western Beqaa.