WASHINGTON (AFP) - Who is the biggest jerk in US diplomacy? America was left guessing after a tantalising near-revelation in the latest batch of Hillary Clinton's e-mails to be made public.

The State Department has been releasing the totality of Clinton's work e-mails from her time as US top diplomat, amounting to tens of thousands of missives peppered with sometimes quirky insights.

"Just for the record, if she does not already know, XXXXX is one of the biggest jerks in the foreign service," the US ambassador to Germany John Kornblum is quoted as saying, in a newly-released e-mail from Clinton adviser Sidney Blumenthal.

Dated March 31, 2011, the partially-redacted message warns Clinton about a possible candidate to succeed William Burns as US under secretary for political affairs.

"Not only can he not get along with people or think clearly on anything, he also went over totally to the dark side during the Bush administration," the then ambassador is quoted as saying of the candidate, whose name is blacked out.

"He once literally shouted me down at a conference where I suggested the Bush administration was hurting US relations with Europe."

The frank missive was part of a batch of 5,500 e-mails made public on New Year's Eve, in line with a court order triggered by Clinton's use of a private email as secretary of state in 2009-2013.

The State Department is required to release all 55,000 pages of Clinton's work-related emails by Jan 20.

Clinton has acknowledged she exclusively used a private e-mail account and private server while secretary of state, opting against a government account despite official recommendations.

The issue erupted into a scandal as the Democratic frontrunner was kicking off her 2016 presidential campaign, and remained a Republican talking point for months.