Democratic Party's presumptive presidential nominee Hillary Clinton is projected to win the final presidential primary in Washington, D.C. ahead of her private meeting with rival Vermont Senator Bernie Sanders.

Washington: Hillary Clinton won the final Democratic primary on Tuesday night as voters in Washington, D.C., delivered a resounding win to the party's presumptive nominee, one week after she clinched the nomination.

The contest marked the close of one of the longest primary fights in American history, Politico reported.

Clinton's rival Vermont Senator Bernie Sanders contested the final primary, even while acknowledging the reality of the delegates maths.

However, Sanders is yet to officially concede, and on Sunday he said he would take his campaign to the Democratic convention in July.

He reiterated at a news conference Tuesday the need to change the Democratic Party, and said he would take "that fight" to the convention in Philadelphia.

Prior to the results, a meeting between the two candidates was held on Tuesday night. Clinton, accompanied by campaign manager Robby Mook and campaign chairman John Podesta, met Sanders, who was joined by his wife Jane and campaign manager Jeff Weaver.

"Sanders congratulated Secretary Clinton on the campaign she has run and said he appreciated her strong commitment to stopping Trump in the general election," said Sanders' spokesman Michael Briggs in a statement.

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

After clinching the nomination on June 7, Clinton also received the endorsements of President Barack Obama and Senator Elizabeth Warren.

Over the course of more than four months, Clinton captured 28 states compared to Sanders' 22, and she won 15.7 million votes in the primaries, compared to 12 million for Sanders.

The contests prior to Washington, D.C., yielded 2,203 pledged delegates for Clinton and 1,828 for Sanders.

A total of 2,383 delegates were needed to capture the party's nomination, and the 581 super delegates supporting Clinton were more than enough to put her over the top.

In 2008, Clinton lost the D.C. primary to then-Senator Obama, who captured 75 per cent of the vote. But the heavily African-American electorate, which proved a disadvantage to Clinton in that contest, helped to deliver victory for the former Secretary of State Tuesday night.

Clinton has dominated among African-American voters, dating back to South Carolina, where their votes made up 61 per cent of the February 27 Democratic primary electorate.

Clinton captured 86 per cent of the African-American vote there.

This final contest comes more than four months after voting began on February 1 with the Iowa caucuses.