John Kerry has been sworn in as US Secretary of State, replacing Hillary Clinton.
Vice-President Joe Biden, swearing in his longtime friend, said: "Few men or women I have ever worked with have more integrity or honour than John Kerry."
Kerry quipped: "I've grown used to being sworn at, it's really nice to be sworn in."
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."