US President Barack Obama has announced his formal endorsement of Hillary Clinton.

In an online video, Obama calls on Democrats to unify behind his former secretary of state's bid and "build on the progress" of his eight terms in the White House.

"I want you to be the first to know that I'm with her," he says.

Obama is also praising Democratic primary candidate Bernie Sanders for engaging young voters and highlighting the issues of income inequality and money in politics. "Embracing that message is going to help us win in November," says Obama.

Clinton and Obama will campaign together in Green Bay, Wisconsin, on June 15.