NEW YORK/WASHINGTON: US presidential candidate Hillary Clinton for the first time apologised on Tuesday for using a private email server at her New York home for her official communications as secretary of state.

"That was a mistake. I'm sorry about that. I take responsibility," Clinton, the front-runner for the Democratic Party's nomination in the 2016 presidential election, said in an interview with ABC News, while still insisting that she violated no rules.

"What I had done was allowed, it was above board," she said. "But in retrospect, as I look back at it now, even though it was allowed, I should have used two accounts. One for personal, one for work-related emails."

In two earlier interviews in the past week, Clinton had declined to apologise for the controversial email arrangement while working as the nation's top diplomat, saying it was allowed by the State Department.

Clinton has been forced to defend herself against growing public scrutiny of how she handled sensitive government information since the email arrangement first came to light in March.

The Federal Bureau of Investigation is now examining the email server to see whether any information, including classified information, was mishandled.

In addition to persistent criticism from opponents and media questions, the State Department's court-ordered monthly releases of the more than 30,000 emails in question has kept the issue in the public eye, roiling Clinton's efforts to nail down the nomination.

Recent polls have found more than half of all voters say she is untrustworthy.

VEXING QUESTION

On Tuesday, US officials said the State Department plans to move about 50 workers into temporary jobs to bolster the office sifting through her emails and grappling with a vast backlog of other requests for information to be declassified.

The move illustrates the huge administrative burden caused by Clinton's decision to use a private email address as secretary of state and a judge's ruling in a Freedom of Information Act (FOIA) lawsuit that they be released.

Separately, Secretary of State John Kerry announced that he was naming Ambassador Janice Jacobs to serve as the State Department's "transparency coordinator" to help the agency

respond to FOIA and congressional requests more efficiently.

One particularly vexing question for Clinton has been whether she may have handled classified information on the private email system, a practice that the government forbids.

Initially, Clinton said she handled no classified information on her unsecured private email server. More recently, she has said she sent and received no information that was "marked" classified, a position she repeated on Tuesday in her interview with ABC.

Citing intelligence officials, the New York Times on Tuesday reported that a special intelligence review of two emails she received as secretary of state had endorsed a finding by the inspector general for the intelligence agencies that the emails contained highly classified information when she received them.

The review by the Central Intelligence Agency and the National Geospatial-Intelligence Agency concluded the emails, including one about North Korea's nuclear program, were "Top Secret," when sent to Clinton in 2009 and 2011, the paper said.

The Clinton campaign and the State Department disagreed with the conclusion and noted that agencies within the government often have different views of what should be classified.