Hillary Clinton declares victory in the U.S. Democratic Presidential Primary on June 7, 2016. [Photo: Xinhua]

U.S. President Barack Obama formally threw his support behind presumptive Democratic presidential candidate Hillary Clinton in a web video Thursday.

"I don't think there's ever been someone so qualified to hold this office," said Obama in the video posted on Clinton's Facebook page.

"I'm with her. I'm fired up and I cannot wait to get out there and campaign with Hillary," Obama said.

According to White House spokesman Josh Earnest, the first joint appearance of Obama and Clinton will take place next Wednesday in Wisconsin, one of the crucial swing states in the general election.

Obama's endorsement came just moments after his meeting with Vermont Senator Bernie Sanders, Clinton's rival in the nomination race, and according to Earnest, Sanders "was not surprised" by Obama's endorsement.

Although he still declined to endorse Clinton at the moment, Sanders told reporters after the meeting that he would do everything "to make sure Donald Trump does not become president of the United States."

As the Democratic primary season was all but over, the notion of party unity had become a crucial topic in the Democratic field.

So far, Sanders had offered mixed response as to what his next step would be after being mathematically eliminated from the race.

Sanders had earlier pledged to continue the fight into the national convention in July when party nomination would be formally announced. However, he later indicated that he would "assess" his path to victory in the wake of California's primary which was held on Tuesday.

Clinton notched an easy victory by two-digit lead in California.

Despite the urgency to unite the party, the White House said on Thursday it was up to Sanders to make the decision as to when and how he ends the campaign.

"Senator Sanders has more than earned the right to make his own decision on his own timeframe about the future of his campaign and the president certainly respects the important work that Senator Sanders has done on the campaign trail," said Earnest in the daily briefing on Thursday.