The Miami Herald said the wording of the announcement in Havana indicates Castro's medical condition is serious.
It is unclear what type of operation the longtime leader of the communist island is scheduled to undergo, CNN said, citing the Cuban television report.
The announcement on state television said Castro suffered from gastrointestinal bleeding, the Herald said. The statement was read by Carlos Valenciaga, Castro's secretary, and linked the condition to stress on Castro from recent appearances in Cuba Argentina, the newspaper said.
U.S. officials have speculated that Castro suffers from Parkinson's disease, an allegation he has denied.
The 79-year-old Castro -- who turns 80 Aug. 13 -- has recently mentioned his younger brother, age 75, would be his likely successor when he steps down or dies.
Raul Castro is currently Cuba's minister of defense.