Leaked emails from top US Democrats have caused friction just days before the party’s national convention.

A cache of emails released by Wikileaks appears to show several senior party figures mocking former presidential contender Bernie Sanders.

Over 19,000 emails from seven officials, covering the period from January 2015 to May 2016, were made public ahead of a crucial week for the party.

Hillary Clinton is expected to formally receive the Democratic nomination for president on Tuesday, during the second day of the Philadelphia convention.

Sanders backed his one-time rival earlier this month after securing a series of progressive changes to the party's platform.

The endorsement was aimed at building opposition to Donald Trump and rallying supporters around the then presumptive Democratic nominee. This latest leak, however, has threatened that image of unity. 

In one email conversation, Democratic National Committee (DNC) chief financial officer Brad Marshall suggested the party could “get someone to ask” an unnamed person, presumed to be Sanders, about his religious belief.

“Does he believe in a God. He had skated on saying he has a Jewish heritage,” he said.

“I think I read he is an atheist. This could make several points difference with my peeps.

“My Southern Baptist peeps would draw a big difference between a Jew and an atheist.”

The DNC’s CEO, Amy Dacey, replied: “AMEN.”

RELEASE: 19,252 emails from the US Democratic National Committee https://t.co/kpFxYDoNyX #Hillary2016 #FeelTheBern pic.twitter.com/nklNO5WSQL

— WikiLeaks (@wikileaks) July 23, 2016

'He isn't going to be president'

The party’s chair, Debbie Wasserman Schultz, wrote in another exchange that Sanders “isn’t going to be president”.

She was responding to an interview in which the socialist candidate said she would be dropped from the position if he were elected to the White House.

Sanders’s campaign manager, Jeff Weaver, told ABC News that the team was “disappointed” by the emails.

"Someone does have to be held accountable," he said. 

"We spent 48 hours of public attention worrying about who in the Trump campaign was going to be held responsible for the fact that some lines of Mrs Obama's speech were taken by Mrs Trump.

“Someone in the DNC needs to be held at least as accountable as the Trump campaign."

Trump quickly jumped on the leak, writing on Twitter that the "vicious" emails "show plans to destroy" Sanders. 

Leaked e-mails of DNC show plans to destroy Bernie Sanders. Mock his heritage and much more. On-line from Wikileakes, really vicious. RIGGED

— Donald J. Trump (@realDonaldTrump) July 23, 2016