Two-year-old Muhammad Ibrahim was more awake than brother Ahmad and seemed to recognize his sibling after he was wheeled in a red wagon into Ahmad's room, said Dr James Thomas, chief of critical care at Children's Medical Center in Dallas, Texas. 

"He was clearly delighted by the ride and when he saw his brother for the first time since surgery he stared at him intently," Thomas said in a statement.

"Those present for the encounter, including his parents, indicated Muhammad appeared to recognize his brother."

A second visit was planned later in the day when Ahmad was expected to be more alert, he said.

The twins before the operation

The visit on Friday came as Ahmad was upgraded from critical to serious condition and began physical therapy. Muhammad was upgraded to serious condition earlier.

It will take several weeks to determine if the boys suffered brain damage in the operation in which a team of five neurosurgeons separated their shared brain material, as well as the shared circulatory systems that fed blood to their brains.

The boys were born in a town 800km south of Cairo, on 2 June 2001.