U.S. Senator Bernie Sanders has endorsed former presidential rival Hillary Clinton as the Democratic nominee ahead of the party's July convention.

"Secretary Clinton has won the Democratic nominating process," Sanders said to cheers in Portsmouth, New Hampshire.

"And I intend to do everything I can to make certain she will be the next president of the United States."

Presumptive Republican nominee Donald Trump issued a statement saying Sanders became part of a "rigged system" by endorsing Clinton.

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