By Arshad Mohammed

WASHINGTON (Reuters) -- The U.S. State Department plans to move about 50 workers into temporary jobs to bolster the office sifting through Hillary Clinton's emails and grappling with a vast backlog of other requests for information to be declassified, officials said on Tuesday.

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

Clinton on Tuesday for the first time apologized for her use of private email, telling ABC News: "That was a mistake. I'm sorry about that." The news channel reported the comment before broadcast of the full interview at 6:30 p.m. EDT.

The extra staff will not work on the monthly, court-ordered release of Clinton emails, which are being handled by about 20 permanent, and 30 part-time, workers, officials said. The new staff will fill in for those workers and may also handle other Clinton FOIA requests.

The front-runner to be the Democratic presidential candidate in the 2016 election has been heavily criticized since it emerged in March that she used the private set-up rather than a government-issued email address.

In a notice to employees on Sept. 2, the State Department advertised for people with skills in coordinating and assessing FOIA requests and deciding if information may be declassified and released to the public.

The notice, a copy of which was obtained by Reuters, is titled "Enhancing Transparency: Immediate Detail Opportunities At State" and calls for workers to apply for reassignment for 9 to 12 months. Applications are due on Thursday and the agency plans to make selections by Sept. 18.

In addition to filling in for workers pulled from their normal duties to handle the crush of work from the Clinton emails, officials said the extra staff would help the department grapple with a surge in FOIA requests more generally, related litigation and a huge backlog of information requests.

On Tuesday, 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.

The agency had an overall backlog of 10,045 FOIA requests at the end of fiscal year 2014 on Sept. 30, up about 15.8 percent from the previous year, according to its FOIA reports.

(Additional Reporting by Jonathan Allen in New York; Reporting By Arshad Mohammed; Editing by Christian Plumb)