US diplomat Richard Holbrooke, the US special envoy to Afghanistan and Pakistan, has died after undergoing a second round of surgery to treat a heart condition, the state department said.
Holbrooke was admitted to George Washington University Hospital in the nation's capital on Friday morning after he fell ill and collapsed at the state department. Surgeons spent more than 20 hours trying to repair a torn aorta, but were unsuccessful.
Barack Obama, the US president, said he was saddened by the death of a man who "served the country he loved with honour and distinction."
"Michelle and I are deeply saddened by the passing of Richard Holbrooke, a true giant of American foreign policy who has made America stronger, safer, and more respected. He was a truly unique figure who will be remembered for his tireless diplomacy, love of country, and pursuit of peace."
Hillary Clinton, the US secretary of state, said the United States had lost one of its "fiercest champions".
"Richard Holbrooke served the country he loved for nearly half a century, representing the United States in far-flung war zones and high level peace talks, always with distinctive brilliance and unmatched determination," Clinton said in a statement.
"He was one of a kind - a true statesman - and that makes his passing all the more painful," she said.
Holbrooke's death is a significant blow to the Obama administration just days before it is scheduled to announce the latest review of US policy in Afghanistan. He was a key member of the team seeking to steer the US on a course of gradually reduced involvement in the country and transfer of responsibility towards the Afghan military forces.
Holbrooke's forceful style earned the 69-year-old nicknames such as "The Bulldozer" or "Raging Bull."
He brokered the 1995 peace agreement that ended the Balkans war, and was a key player in Obama's efforts to turn around the faltering nine-year-old war in Afghanistan.
Holbrooke served as the US ambassador to the United Nations during the Clinton administration. He was also an ambassador to Germany from 1993 to 1994 and then assistant secretary of state for European affairs.
Holbrooke was once called "Washington's favourite last-ditch diplomat'' and "America's toughest diplomatic tactician'' by Time magazine, a US publication. He was nominated for the Nobel Peace Prize seven times.
Holbrooke joined Obama's administration in 2009 as special representative to Afghanistan and Pakistan, which involved co-ordinating the approach to trouble spots that are key foreign policy priorities for Obama.
He had been very critical of former president George Bush's Afghanistan policy, and his position Obama administration was considered critical as the new president sought to crackdown on al-Qaeda and Taliban in the region.
A torn aorta is a condition in which a rip develops in the inner wall of the body's largest artery, allowing blood to enter the vessel wall and weaken it.
The problem can lead to rapid death if not corrected. It causes serious internal bleeding, a loss of normal blood flow and possible complications in organs affected by the resulting lack of blood, according to medical experts.