Hillary Clinton formally resigns as US Secretary of State

Hillary Clinton has formally resigned as the US Secretary of State saying she was "honoured to serve" in President Barack Obama's administration.

In a letter sent to the President shortly before she left the State Department for the last time, she said: "I am more convinced than ever in the strength and staying power of America's global leadership and our capacity to be a force for good in the world."

Clinton will be replaced by former Democratic presidential nominee John Kerry who will take the oath of office in a private ceremony today.