The 15 members of the UN Security Council are expected to begin closed-door talks on North Korea in the coming minutes.
The scheduled meeting in New York is to discuss a draft resolution that would sanction the country for its third nuclear test last month.
UN diplomats have told Reuters new agency they expect to vote on the draft resolution by the end of this week, although it could face opposition from China.
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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.