US Secretary of State Hillary Clinton fainted and suffered a concussion while battling a nasty stomach bug, an official said Saturday, just days before the release of a key probe into the Benghazi attack.

The news came just as Clinton, 65, had been expected to testify on Thursday to US lawmakers about the findings of the investigation into September's militant attack on the US consulate in Benghazi, Libya.

The normally indefatigable Clinton, who in her four years as the top US diplomat has traveled almost a million miles visiting 112 countries, was "recovering" but plans to stay away from the office next week, her top aide Philippe Reines said.

"While suffering from a stomach virus, Secretary Clinton became dehydrated and fainted, sustaining a concussion," he added in a statement, without giving further details of what happened.

Her doctors said Clinton had experienced "extreme dehydration, and subsequently fainted. Over the course of this week, we evaluated her and ultimately determined she had also sustained a concussion."

Doctors Lisa Bardack, from Mount Kisco Medical Group, and Gigi El-Bayoumi of George Washington University said they had recommended "she continue to rest and avoid any strenuous activity, and strongly advised her to cancel all work events for the coming week."

They added that they would continue to monitor her progress.

Representative Ileana Ros-Lehtinen, who chairs the House Foreign Affairs Committee, wished Clinton a quick recovery.

"It is, however, unfortunate that Secretary Clinton is unable to testify next week... on the investigation into the terrorist attack that killed four Americans and left others injured," Lehtinen added.

She said lawmakers still have "tough questions about State Department threat assessments and decision-making on Benghazi. This requires a public appearance by the secretary of state herself."

Clinton has already been off work for the past week, after canceling a trip to North Africa when she contracted the stomach virus on her return from a five-day European tour.

She had been due to testify at open hearings next week in both the House and Senate on the outcome of the State Department investigation into the Benghazi assault in which the ambassador and three other Americans died.

State Department acting deputy spokesman Patrick Ventrell confirmed Friday the report should be completed by early next week.

Its findings are not binding, but "it's a chance for the department as a whole to look at our operations and look at what needs to be done to improve security," he said.

Clinton has said she takes full responsibility, repeatedly stressing that no one wants to find out what happened in Benghazi more than she does.

The September 11 attack became a political punchbag in the final, furious weeks of the 2012 presidential elections, and has already complicated Obama's calculations for his second term cabinet.

The report's findings are also likely to ignite another storm of protest, with Republicans already scenting blood.

Republicans have castigated President Barack Obama's administration for failing to provide proper security, and for employing local Libyan security staff.

They also allege the administration sought to cover up links to Al-Qaeda by initially insisting the attack was sparked by a protest over an anti-Islam video, which had triggered public fury in Libya's neighbor Egypt.

Veteran diplomat Thomas Pickering, who has been heading up the accountability review board and another member of the team, Admiral Mike Mullen, will instead brief the House panel Wednesday behind closed doors.

Even though Clinton aims to step down as secretary of state early next year, Obama has not yet revealed his pick to replace her.

US Ambassador to the United Nations Susan Rice on Thursday abandoned her hopes of succeeding Clinton, after coming under Republican fire for saying in the days after the assault that, according to the available intelligence, it resulted from a "spontaneous" demonstration.

Rice asked Obama to withdraw her name from consideration, saying it would only lead to a protracted fight after several Republicans threatened to block her nomination.

Veteran Senator John Kerry, the chairman of the Senate Foreign Relations Committee, is now the odds-on favorite to replace Clinton at the helm of the State Department -- an ambition he has long cherished.