U.S. diplomat Richard Holbrooke, shown in Kabul on Oct. 27, faces a lengthy recovery from surgery for a torn aorta. ((Omar Sobhani/Reuters))

Veteran U.S. diplomat Richard Holbrooke has received phone calls from the presidents of Afghanistan and Pakistan as he recovers from lifesaving surgery to fix a tear in the large artery that moves blood from his heart.

Holbrooke, 69, underwent additional procedures Sunday to improve blood circulation, but remained in critical condition at George Washington University Hospital.

The president's diplomatic point man on the Afghanistan war, Holbrooke was stricken Friday while at the State Department and was rushed to the hospital, where he underwent more than 20 hours of surgery to repair the tear and bleeding in his aorta.

The State Department said Sunday he received calls from Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari. As U.S. President Barack Obama's special envoy to Afghanistan and Pakistan, the longtime diplomat has made numerous visits to the region.

Holbrooke was meeting with U.S. Secretary of State Hillary Clinton about midmorning Friday when he fell ill, collapsed and was rushed to the hospital a few blocks away. While doctors were said to have stabilized Holbrooke's condition, recovery is likely to be lengthy.

"Many people would have succumbed to that. Richard is fighting through," Obama adviser David Axelrod told CNN's State of the Union on Sunday. "Anyone who knows him, and I was with him Friday morning before this happened, knows how tough and resilient he is, and we're all praying that that quality sees him through now."

A torn aorta involves a rip in the inner wall of the body's largest artery, allowing blood to enter the vessel wall and weaken it. If not corrected, the condition could lead to rapid death.

As blood enters the wall, it reduces blood flow, leading to serious internal bleeding, a loss of blood flow and possible complications in organs affected by the lack of blood, according to medical experts.