Veteran US diplomat Richard Holbrooke, 69, the US special envoy for Afghanistan and Pakistan, has died after undergoing surgery after suffering a torn aorta, US media reported on Monday.
President Barack Obama had told members of Holbrooke's family at a diplomatic holiday reception at the State Department just hours earlier: "America is more secure and the world is a safer place because of the work of Ambassador Richard Holbrooke.
Earlier, secretary of state Hillary Clinton said in Canada that Mr Holbrooke, who brokered the Dayton Accords, that ended the Bosnian war, remained in stable but "very critical condition."
After a career spanning nearly 50 years at the pinnacle of US diplomacy, Mr Holbrooke fell ill on Friday, during a meeting at the State Department.
A hard-nosed trouble shooter, Mr Holbrooke is perhaps best known for brokering the 1995 peace agreement that ended three years of war in Bosnia.
As a special US envoy in the current Afghan conflict, he has had the daunting task of pushing Kabuland Islamabad to work together against resurgent Al Qaeda and Taliban militants in Afghanistan and Pakistan.
Mr Obama called him "a towering figure in American foreign policy, a critical member of my Afghanistan and Pakistan team, and a tireless public servant who has won the admiration of the American people and people around the world."
His health problems come at a critical time for US policy in the region, with the administration due to conduct a review of its troop surge in Afghanistan and campaign against the Taliban on both sides of the Afghan-Pakistan border.
US defence officials have said they do not expect a change in course as a result of the review, seeing improvements in the security situation even though government corruption and Pakistani reluctance to go after insurgent safe havens remain big problems.
Hard-charging and impatient, Holbrooke has maintained a hectic travel schedule, and was in Islamabad as recently as last month.
His health has at times been a concern. He underwent tests in New York in April for possible blocked arteries, though doctors gave him the all-clear to travel.
He maintained a lower profile in his latest assignment working with difficult allies in Afghanistan and Pakistan than when he brokered the agreement that ended the Bosnian war.
Dubbed "the bulldozer", he alternately browbeat and cajoled the nationalist leaders of formerYugoslavia until he succeeded in forging the peace deal in November 1995 in Dayton, Ohio, following a round of Nato air strikes against Serb forces.
The Dayton agreement has held the shaky Bosnian state together despite persistent tensions among rival Muslim, Serb and Croat communities.
Mr Holbrooke often has been spoken of as a future secretary of state, but was passed over by Mr Obama in favour of Hillary Clinton, his former rival for the presidency