The State Department said Saturday that after suffering from a stomach virus, Secretary Hillary Clinton fainted from dehydration and sustained a concussion.
Clinton is recovering and will continue to work from home next week, the agency said, while being monitored regularly by her doctors.
The secretary had already canceled an important overseas trip because of her illness this past week. Clinton was expected to meet with Syrian rebels in Morocco on Wednesday, after President Obama said the United States would formally recognize opposition groups as a legitimate authority. Deputy Secretary of State William Burns instead replaced Clinton on the trip.
"This is quite extraordinary because the secretary, as you know, works nonstop, has traveled more than anyone else, and had come home from a very difficult trip," NBC News' Andrea Mitchell said to msnbc host Alex Witt on Saturday.
According to NBC News' Kristen Welker, senior administration officials say Clinton fainted days ago, information that only now has been made public.
"It's been very, very difficult to learn about her health because officials have been very, very private about it," Mitchell later added.
Clinton was scheduled to appear before committees in both the House and Senate this coming week, to testify on the attack on the U.S. consulate in Benghazi, Libya from September. The details of the attacks have been a contentious topic--particularly for Republicans questioning the Obama administration's handling of the attacks. GOP criticism about her comments on the attack led U.N. Ambassador Susan Rice to withdraw her name for consideration to replace Clinton as secretary of state.
"I am sorry to learn of Secretary Clinton's ill health and I wish her a quick and full recovery," Rep. Ileana Ros-Lehtinen, R-Fla., said in a statement Saturday. "It is, however, unfortunate that Secretary Clinton is unable to testify next week before the House Committee on Foreign Affairs on the investigation into the terrorist attack that killed four Americans and left others injured."
Clinton instead will be replaced by Deputy Secretaries Burns and Thomas Nides.