Hillary Clinton says she wished she had made a "different choice" and not used a private email account while serving as US secretary of state.

Democratic presidential hopeful Hilary Clinton has hit back at one of her Republican rivals, Jeb Bush, over who is responsible for instability in Iraq.

It was billed by some as a preview of next year's general election showdown - the leading lights of two political dynasties offering duelling speeches from the same stage in the key battleground state of Florida.

Ellen DeGeneres has become the latest high-profile figure to join the tens of thousands of people responding to a powerful post on the Humans of New York Facebook page.

A longtime friend of Hillary Clinton gave her detailed advice on issues ranging from British politics to Afghanistan and Iran while she was secretary of state even though he was not employed by the US government, according to emails released on Tuesday.

Hillary and Bill Clinton have earned at least $30 million since January 2014, including more than $25 million for delivering about 100 speeches, according to a government filing.