Syrian state news agency SANA has said 44 people have died after car bombs ripped through Jaramana, a eastern suburb of Damascus.
The district is populated mostly by Christian and Druse, a minority sect, and is mostly loyal to President Assad.
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.