Democratic US presidential candidate Bernie Sanders greets supporters on Hollywood Boulevard in Hollywood, Los Angeles, California, US, June 7, 2016. [Photo/Agencies]

Sanders was determined to stay in the race, even after the Associated Press and NBC reported on Monday night that Clinton had clinched the number of delegates needed to win the nomination. A Sanders campaign spokesman castigated what he said was the media's "rush to judgment."

Under Democratic National Committee rules, most delegates to the July 25-28 convention in Philadelphia are awarded by popular votes in state-by-state elections, and Clinton has a clear lead in those pledged delegates.

But the delegate count also includes superdelegates, party leaders who can change their minds at any time. Clinton's superdelegate support outnumbers Sanders' by more than 10 to 1.

The Sanders' campaign has said it can still persuade superdelegates to switch to him, although in practice superdelegates who have announced their intentions are unlikely to change their minds.

Sanders would have to get more than 60 percent of the superdelegates backing Clinton to switch their votes. So far, his campaign manager, Jeff Weaver, acknowledged they had not converted a single delegate.

Trump, 69, became his party's presumptive nominee last month, outlasting 16 Republican challengers.