The Aug. 3 operation was to remove a "small mass" from his liver, according to The Associated Press. After the procedure, Carter's prognosis for a full recovery was initially thought to be "excellent," a spokeswoman for his nonprofit The Carter Center told the AP.
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President Barack Obama sent his best wishes for a fast, full recovery Wednesday afternoon. "Jimmy, you're as resilient as they come, and along with the rest of America, we are rooting for you," he said, in a statement.
Carter has stayed active with his nonprofit, which was founded in 1982 and focuses on health care and human rights, among other initiatives. Health issues have affected some of his work this year, like a May trip to Guyana that was cut short by a bad cold.
He recently completed a tour for a new book, "A Full Life: Reflections at Ninety."
Carter won the Nobel Peace Prize in 2002 for what the prize committee called "decades of untiring effort to find peaceful solutions to international conflicts." The native of Plains, Georgia, was the state's governor before his election to the White House.
—The Associated Press contributed to this report