(MSNBC) — A U.S. graduate student who was attacked by two chimpanzees while leading a tour at a sanctuary for abused animals underwent six hours of surgery on Sunday at a hospital in South Africa.

Andrew Oberle, 26, suffered extensive soft-tissue injuries to his arms and legs. His wounds were cleaned and stitched under induced sedation, said officials at Mediclinic Nelspruit hospital in Nelspruit, a city east of Johannesburg. He remained in critical but stable condition.

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Oberle, a University of Texas at San Antonio graduate student, was mauled Thursday by two large male chimpanzees who pulled him under a fence into their enclosure at the Jane Goodall Institute Chimpanzee Eden near Nelspruit.

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