John Kerry hailed the contribution of outgoing Secretary of State Hillary Clinton at his official swearing in ceremony in Washington.
"I want to thank Secretary Clinton for the unbelievably high standard of energy, commitment and vision that she set in her superb stewardship of the US State Department," said Kerry.
The new Secretary of State also paid tribute to Barack Obama, saying: "The President has restored American's place and our reputation in the world."
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The UN has said it is deeply concerned by reports hundreds of Syrian men have disappeared amid a mass exodus from rebel-held areas.
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Another mild day with highs reaching up to 13C degrees in mild southerly winds