US President Barack Obama has said that he and his wife are praying for the recovery of envoy Richard Holbrooke, who has undergone heart surgery.
He described the US envoy to Afghanistan and Pakistan as a "towering figure" in American foreign policy, and a key member of his team.
Mr Holbrooke, 69, underwent lengthy surgery on Saturday after falling ill at work in the state department.
He was said to be in a stable but critical condition in hospital.
His family were reported to be at his bedside in the US capital's George Washington University Hospital.
Mr Holbrooke served as an envoy to several Democratic presidents but it was his role in bringing the war in Bosnia to an end in 1995 that gained him wider recognition.
After taking office in January 2009 President Obama named him as his special representative to Afghanistan and Pakistan.
Since then he has played a key role in defining the strategy the Obama administration pursues in the region.
Mr Holbrooke was admitted to hospital on Friday and had surgery to repair a tear in his aorta, the largest artery in the human body, which carries oxygenated blood from the heart.
A torn aorta is a condition in which a tear develops in the inner wall of the artery, allowing blood to enter the vessel wall. It can kill quickly if left untreated.
"We continue to pray for his recovery, and support his family in this difficult time," Mr Obama said in a statement.
He described the veteran diplomat as a "tireless public servant who has won the admiration of the American people and people around the world".
Secretary of State Hillary Clinton and Adm Mike Mullen, chairman of the Joint Chiefs of Staff, have both visited Mr Holbrooke in hospital, Mr Obama said.
Nicknamed The Bulldozer, Mr Holbrooke has gained a reputation for confronting warring leaders to get them to come to the negotiating table.
He is one of the state department's top diplomats, having served in Vietnam and at the UN.
In the wake of his success over Bosnia, he was nominated for the Nobel Peace Prize.
In 1997, then President Bill Clinton named him as ambassador to the UN.
In his role as President Obama's envoy, he has clashed regularly with Afghan president Hamid Karzai.
He has maintained a hectic travel schedule. In September, he visited Pakistan to see the aftermath of the floods which devastated the country.
He has also travelled to the north-west of the country, a stronghold for insurgents which lies largely outside government control.
Mr Holbrooke was born in New York and is of German-Jewish descent. He was educated at Brown University, and is married to the writer Kati Marton.
As well as his diplomatic career, Mr Holbrooke has also worked in the financial sector.