Richard Holbrooke, the US envoy to Afghanistan and Pakistan, has been taken critically ill, the US state department says.
Mr Holbrooke, 69, was admitted to hospital on Friday and has undergone surgery to repair a tear in his aorta.
His family have joined him at his hospital bedside in Washington DC.
Mr Holbrooke is best known for brokering the Dayton peace accords in 1995, which ended the Bosnian war.
President Barack Obama named Mr Holbrooke as his special representative to Afghanistan and Pakistan in January 2009.
Nicknamed “the Bulldozer”, Mr Holbrooke gained a reputation for confronting warring leaders to get them to come to the negotiating table.
Mr Holbrooke is one of the state department’s top diplomats, having served in Vietnam and at the United Nations.
He has clashed regularly with Afghan president Hamid Karzai.
After the disputed presidential elections in August 2009, Mr Holbrooke was widely reported to have confronted Mr Karzai about poll irregularities.
However, a spokeswoman for the US embassy in Kabul denied there had been any shouting or that Mr Holbrooke had stormed out.
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