BEIRUT, June 20 (Reuters) - A suicide Bomber blew himself up at a security checkpoint in Leba**n's Eastern Bekaa Valley on Friday, Lebanese police said. The checkpoint is situated on the main highway joining Leba**n and Syria's capitals, in an area where Sunni Muslim militants fighting to topple Syrian President Bashar al-Assad have been active. (Reporting by Lalia Bassam, Writing by Sylvia Westall, Editing by Samia Nakhoul)