Obama made the "unusually candid remarks" during a top Democratic fund raiser in Austin, Texas last week, media reported.

"Mr. Obama chose his words carefully, and did not explicitly call on Mr. Sanders to quit the race, according to those in the room," it said.

Clinton's sole primary rival Senator Bernie Sanders is now lagging behind in delegate count to be the party's nominee for the November 8 Presidential elections while the Democrat frontrunner had a clear sweep of the March 15 primaries in Florida, Illinois, North Carolina, Ohio and Missouri.

White House Press Secretary Josh Earnest acknowledged Obama's comments.

"President Obama made a case that would be familiar to all of you, which is that as Democrats move through this competitive primary process, we need to be mindful of the fact that our success in November in electing a Democratic president is dependent on the commitment and ability of the Democratic Party to come together behind our nominee," Earnest told reporters at his daily news conference.

However, Earnest said Obama "did not indicate or specify a preference in the race".

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