Charlie Neibergall, Associated Press
FILE - In this Sept. 22, 2015 file photo, Democratic presidential candidate Hillary Rodham Clinton speaks in Des Moines, Iowa. The State Department published about 6,000 additional pages of Hillary Rodham Clinton’s emails, covering a timespan when U.S. diplomacy was rocked by the leaking of thousands of confidential cables by the anti-secrecy website WikiLeaks and the outbreak of the Arab Spring.

WASHINGTON — The State Department released on Wednesday 6,300 additional pages of Hillary Rodham Clinton's emails, covering a period when U.S. forces killed Osama bin Laden and the Arab Spring rocked American diplomacy.

The release is the latest in the agency's rolling production of emails chronicling Clinton's tenure as secretary of state and canvasses the major U.S. foreign policy upheavals of 2011. All of the messages were written by Clinton or sent by others to the private email account she used as America's top diplomat — a months-old revelation that continues to hamper her campaign for the Democratic presidential nomination.

Thirty-seven percent of Clinton's 30,000 work-related emails are now public. The State Department plans to release them all by January.

Clinton has faced increasing questions over whether her unusual email setup, which included a homebrew server at her New York home, amounted to a proper form of secrecy protection and records retention. The emails themselves — many redacted heavily before public release — have provided no shocking disclosures thus far.

The former first lady and New York senator had maintained that nothing was classified in the correspondence, but the intelligence community has identified messages containing "top secret" information. Clinton had insisted that all of her work emails were being reviewed by the State Department, but Pentagon officials recently discovered a new chain of messages between Clinton and then-Gen. David Petraeus dating to her first days in office that she did not send to the State Department.

As part of Wednesday's release, officials upgraded the classification level of portions of 215 emails, State Department spokesman John Kirby said. Almost all were "confidential," the lowest level of classification. Three emails were declared "secret," a mid-tier level for information that could still cause serious damage to national security, if made public.

"The information we upgraded today was not marked classified at the time the emails were sent," Kirby stressed. "It has been subsequently upgraded."