Former of Secretary of State Henry Kissinger underwent a procedure at a New York hospital Tuesday to replace an aortic valve.

New York-Presbyterian Hospital said in a statement Kissinger was “resting comfortably” after the procedure, but gave no other details.

The 91-year-old served under presidents Richard Nixon and Gerald Ford. He was awarded the Nobel Peace Prize in 1973 and the Presidential Medal of Freedom in 1977.

The Associated Press contributed to this report