Hillary Clinton has received commitments from enough delegates to clinch the Democratic presidential nomination, according to the Associated Press and US networks, ensuring she will be the first woman to lead a major US party in the race for the White House.

Clinton reportedly reached the key number one day before Democratic primaries in six US states, including California where she is campaigning heavily, and eight years after she came up short in her nomination battle against Barack Obama.

The former secretary of state now becomes the presumptive Democratic nominee, and will do battle in the general election with Donald Trump, who secured the same status within the Republican camp last month.

Clinton's campaign appeared to acknowledge the tallies that pushed her beyond the 2,383 delegates necessary to clinch the nomination, but said her primary battle with Senator Bernie Sanders was not yet over.

"This is an important milestone, but there are six states that are voting Tuesday, with millions of people heading to the polls, and Hillary Clinton is working to earn every vote," her campaign manager Robby Mook said in a statement.

"We look forward to Tuesday night, when Hillary Clinton will clinch not only a win in the popular vote, but also the majority of pledged delegates."

Sanders immediately responded, insisting no Democrat will lock in the nomination until the Democratic national convention in July because unbound super-delegates cannot vote until then.

"It is unfortunate that the media, in a rush to judgement, are ignoring the Democratic National Committee's clear statement that it is wrong to count the votes of super-delegates before they actually vote at the convention this summer," Sanders spokesman Michael Briggs said in a statement.

Sanders has said he will spend the coming weeks seeking to convince many of Clinton's super-delegates that he is the stronger candidate to go up against Trump.

Clinton came to the brink of the nomination Sunday when she won the US territory of Puerto Rico, pushing her delegate count to 2,373, according to a CNN tally.

She surpassed the threshold Monday after a number of super-delegates -- current and former elected officials and political activists who are not bound to vote for a specific candidate -- recently committed to back her candidacy, according to the AP.

Clinton has earned support from 561 super-delegates, while Sanders has just 47, according to CNN.

Sanders stands at 1,568 delegates, more than 800 behind his rival, and there is no precedent for a massive number of super-delegates switching sides.

Clinton noted on Monday that she has received three million more votes than Sanders, and far more pledged delegates than Sanders as well.