| Holbrooke, left, has been challenged by many tough obstacles in South Asian geopolitics [Reuters]
Richard Holbrooke, the US special representative for Afghanistan and Pakistan, is in critical condition following surgery to repair a tear in his aorta, state department officials have said.
Holbrooke, 69, felt ill at the US state department on Friday and then went to the hospital.
PJ Crowley, state department spokesman, said Holbrooke began to feel unwell while on the department's seventh floor, where the office of Hillary Clinton, the secretary of state, is located.
A department official says Clinton visited Holbrooke on Friday night and again on Saturday.
Within days of taking office in January 2009, US President Barack Obama named Holbrooke, best known for brokering the Dayton peace accords that ended the war in Bosnia, as his special representative to Afghanistan and Pakistan.
A decade after invading Afghanistan to topple the Taliban regime that harboured al-Qaeda, nearly 100,000 US troops remain in the country fighting an insurgency nurtured in part by its ability to find sanctuary in neighbouring Pakistan.
Holbrooke, who has worked as an executive in the financial sector when not at the state department, was said to be a candidate for secretary of state before the job went to Clinton.
His meteoric career included stints in Vietnam as well as serving as the top US diplomat for East Asia, for Europe and at the United Nations.
Holbrooke's most notable achievement was bringing all sides in the Bosnia conflict to the negotiating table at an air base in Dayton, Ohio. The resulting 1995 Dayton accords ended the conflict.
Holbrooke has been a key player in Obama's efforts to turn around the faltering 9-year-old war in
Afghanistan, where violence is at its highest levels since 2001 and at least 477 US soldiers have been killed this year alone.
A prolonged absence for Holbrooke could be a blow to Obama's efforts to demonstrate swift progress in Afghanistan next year, when Washington hopes to start putting Afghan forces in the lead and start bringing US troops home in July.
A frequent visitor to both Afghanistan and Pakistan, Holbrooke has personal relationships with the region's leaders while at home he has sought to allay concerns in the US Congress over the course of the war.
Holbrooke's sudden illness comes just before the White House is expected to roll out next week an assessment of the revised strategy for the two troubled countries that Obama unveiled a year ago.
The review is expected to conclude that despite entrenched corruption and weak governance, US-led forces are making progress on security in parts of Afghanistan.
During congressional testimony on July 28, Holbrooke conceded that fighting a resurgent Taliban and helping to rebuild Afghanistan were massive tasks. But he repeatedly defended the Obama administration's strategy.
He called the Afghanistan mission "the most difficult job I've had in my career".
But, he said, "Number one, on a personal note, I wouldn't be in this job if I thought it was impossible to succeed".