Bernie Sanders hasn't given up just yet

Hillary Clinton might have already started celebrating victory in the race for the Democratic nomination for the US presidency, but Bernie Sanders hasn't given up yet.

"I know the fight in front of us is a very, very steep fight," he said, addressing a crowd in California, "but we will continue to fight for every vote and for every delegate we can. Next Tuesday we continue the fight in the last primary in Washington DC.

"And then we take our fight for social, economic, racial and environmental justice to Philadelphia."

Philadelphia is where the Democratic convention will be held in late July to formally nominate the party's candidate to challenge Republican Donald Trump for the White House.

Sanders appears to be ignoring those who say Clinton has already won as she has surpassed the number of delegates necessary to clinch the nomination.

Tuesday was the last major day of the 2016 primary calendar, with six states voting.

The former secretary of state extended her delegate lead with wins in New Jersey, New Mexico and South Dakota, while Sanders won North Dakota and Montana, US networks said.

Sanders graciously acknowledged Clinton's victories, drawing some boos from the crowd.

"Our fight is to transform this country and to understand that we are in this together. To understand that all of what we believe is what the majority of the American people believe," Sanders said.

Sanders by all accounts has conducted an extraordinary grassroots campaign.

His supporters are deeply passionate, and Clinton has made it clear she would welcome their backing as she seeks to defeat Trump.

Sanders signalled he wants to have a role not just in keeping the White House in Democratic hands, but in shaping the party's policy platform.

"We understand that our mission is more than just defeating Donald Trump, it is transforming our country."