Secretary of State Hillary Clinton will not be able to testify before House and Senate committees next week on the Benghazi issue due to a concussion, the State Department said Saturday.

Clinton was a scheduled to testify Thursday on the Sept. 11 terror attacks on the U.S. Consulate in Benghazi, Libya, in which U.S. Ambassador Christopher Stephens and three other Americans were killed.

Deputy Assistant Secretary of State Philippe Reines said Saturday that Clinton is suffering from a stomach virus, fainted and hit her head, which caused the concussion. said Clinton is suffering from a stomach virus, fainted and hit her head, which caused the concussion.

However, the agency did not say when the fall occurred.

On Saturday afternoon, the House Foreign Affair Committee announced the hearing will proceed Thursday, with or without Clinton.

More On This...

The committee also said former Ambassador Thomas Pickering and retired Adm. Mike Mullen, former chairman of the Joint Chiefs of Staff, would appear Wednesday before members for a classified briefing about Benghazi.

However, committee Chairwoman Rep. Ileana Ros-Lehtinen made clear she still wanted Clinton to testify at some point.

“Although I respect Bill and Tom, we still don’t have information from the Obama administration on what went so tragically wrong in Benghazi,” the Florida congresswoman said Saturday. “This requires a public appearance by the secretary of state herself.”

Senior State Department officials William Burns and Thomas Nides are expected to take Clinton's place at both hearings.

The House and Senate committees said Wednesday that Clinton would testify Thursday.

The State Department over the next two days appeared to waffle on the whether Clinton would testify on the appointed date.

Spokeswoman Victoria Nuland said Thursday the agency’s Accountability Review Board report on the attacks, upon which Clinton will largely base her testimony, might not be complete. However, acting deputy spokesman Patrick Ventrell said Friday he anticipated the report will be ready and Clinton will make the scheduled appearance.

Congressional Republican have led the charge for answers regarding the Benghazi situation -- from security prior to the attacks, to what sparked the deadly violence to what official told the American public in the aftermath.

U.N. Ambassador Susan Rice said five days after the incident, during a round of Sunday talk shows, the attacks appeared to be a “spontaneous” response to early violence related to an anti-Islamic video.

The administration later said the attacks appeared to be terror related.

Rice acknowledged the information she gave Sept. 16 was incorrect but has stuck to her position that she was working off information given to her by intelligence officials and that the information at the time was evolving.

The 65-year-old Clinton on Monday backed out of trip to North Africa and the Persian, citing the illness that she thinks she caught during a recent visit to Europe. Clinton plans to step down in January. She has said she is tired from the job’s long, hectic schedule and needs a rest.

Sen. John Kerry, chairman of the Senate Foreign Affairs Committee, upon being told Saturday about Clinton’s illness and injury “insisted” she not appear at the hearing, said Jodi Seth, spokeswoman for the Massachusetts Democrat.

The Associated Press contributed to this report.