Biden, 45, chairman of the Senate Judiciary Committee and a former Democratic presidential contender, was “awake, alert and progressing satisfactorily,” said Pete Esker, a spokesman for Walter Reed Army Medical Center. “He’s as comfortable as possible.”

Surgeons on Friday afternoon slipped a tiny metal clip around the aneurysm, which is a sac formed at a weak spot on an artery wall. The danger is that, if untreated, the sac could burst.

A neurosurgeon, one of several around the United States consulted by Biden’s surgeons, told United Press International that the senator’s aneurysm had hemorrhaged.

Esker said Biden’s aneurysm was within his cranium but underneath the brain.

Biden, a three-term senator, had been complaining for a couple of weeks about what had been diagnosed as a pinched nerve in his neck.

Biden aide Pete Smith said the senator was listed in “seriously ill” condition but was expected to suffer no significant aftereffects from the surgery.

“The length of his hospital stay will depend on his continued progress,” Smith said.

“I am told that he is in intensive care, which Walter Reed said is normal for surgical patients, and his family is with him at the hospital,” Smith said.