Special Representative for Afghanistan and Pakistan Richard Holbrooke, 69, is in critical condition after a surgery to repair a tear in his aorta.
State Department spokesman P.J. Crowley says doctors completed the surgery this morning, and that he is being joined by his family at George Washington University Hospital in Washington, D.C.
According to Democratic sources, Ambassador Holbrooke, Secretary Clinton's top aide for the region, gasped and was clearly undergoing a medical situation when he became ill yesterday. He is said to have walked out of her office on his own power and was tended to by medical personnel at the State Department before being transported to the Hospital.
Holbrook's Hospitalization comes just days before the Obama administration is set to roll out its Afghanistan review on Thursday; Holbrooke was expected to be part of the public presentations. Aides tell ABC News that the review is complete and his absence should not affect its release.
Ambassador Holbrooke was named to his current post just days after President Obama took office in January 2009 and was immediately tasked with overhauling the United States' efforts in Afghanistan and Pakistan. He is known best for his bombastic personality and stamina, having traveled to the region numerous times since taking office.
The veteran diplomat is a former two-time Assistant Secretary of State, Ambassador to Germany, and was a key player in the Dayton Peace accords, which brokered an end to the conflict in Bosnia. He began his long career as a foreign service officer at the U.S. embassy in Saigon during the Vietnam War.
ABC News' Jake Tapper and Ryan Creed contributed to this report.