Hillary Clinton wins final primary, now has 2,800 delegates

Clinton, 68, a former Secretary of State and a former Senator from New York, is the first women to become a presidential candidate of a major political party in US electoral history.

Clinton received 78.9 per cent of the votes while Sanders could garner only 21.1 per cent.

The result heralds the start of an epic race to the White House between Clinton and brash real estate tycoon Trump who celebrated his 70th birthday today.

"We just won Washington, DC! Grateful to everyone who voted," Clinton tweeted though Sanders is yet to drop out of the race as the two met today.

Clinton secured enough delegates to clinch the Democratic presidential nomination last week, but Sanders declined to drop out and pledged to give every voter a chance to decide between the two candidates.

Sanders privately met Clinton at the Capital Hilton, just after she won the final contest on the Democratic calendar.

The meeting came as the Vermont senator has begun signalling that his campaign is soon to close, though Sanders' aides say he is not expected to immediately endorse Clinton.

"Sanders and Secretary Clinton met in Washington...and had a positive discussion about how best to bring more people into the political process and about the dangerous threat that Donald Trump poses to our nation," said Michael Briggs, Sanders' spokesman, in a statement.

"Sanders congratulated Secretary Clinton on the campaign she has run and said he appreciated her strong commitment to stopping Trump in the general election," it said.

It added that the two discussed a variety of issues where they are seeking common ground: substantially raising minimum wage; real campaign finance reform; making health care universal and accessible and making college affordable and reducing student debt.

"Sanders and Clinton agreed to continue working to develop a progressive agenda that addresses the needs of working families and the middle class and adopting a progressive platform for the Democratic National Convention," the statement said.

Clinton now has 2,800 delegates out of 4,763 delegates. Sanders has 1,832 delegates in his kitty.

Ending the primary elections on a winning note puts the former first lady on a sound footing ahead of the general election in November and the Democratic Party's convention in Philadelphia in July.