"It's a very sensitive area, so we started to pay a lot of attention to that," he said, adding that he had reduced his public appearances.
Chavez made his most recent trip to Cuba on the night of Nov. 27, saying he would receive hyperbaric oxygen treatment. Such treatment is regularly used to help heal tissues damaged by radiation treatment.
Chavez said that he has been coping with pain and that while he was in Cuba thorough exams detected the recurrence of cancer.
He arrived back in Caracas on Friday after 10 days of medical treatment in Cuba, but until Saturday night had not referred to his health. His unexplained decision to skip a summit of regional leaders in Brazil on Friday had raised suspicions among many Venezuelans that his health had taken a turn for the worse.
Chavez said that he was requesting permission from lawmakers to travel to Havana.
"I hope to give you all good news in the coming days," said Chavez, who held up a crucifix and kissed it. "With the grace of God, we'll come out victorious."
Dr. Carlos Castro, scientific director of the League Against Cancer in neighboring Colombia, told The Associated Press that he expects the operation will likely be followed by more chemotherapy.
"It's behaving like a sarcoma, and sarcoma doesn't forgive," Castro said, adding that he wouldn't be surprised if the cancer had also spread to the lungs or other areas.
"We knew this was going to happen," he said. "This isn't good."
Throughout his treatment, Chavez has kept secret various details about his illness, including the precise location of the tumors and the type of cancer. He has said he travels to Cuba for treatment because his cancer was diagnosed by doctors there.
Dr. Michael Pishvaian, an oncologist at Georgetown University's Lombardi Cancer Center in Washington, said in a phone interview that he wasn't surprised by the news.
"I think this is recurrent cancer that at this point is almost certainly not going to go away," Pishvaian said. "It's unlikely that what he's going through now is curable."
He speculated that given what Chavez has said about his cancer, it is most likely a soft-tissue sarcoma. He said those in the pelvis area have a likelihood of recurring of 50 percent to 70 percent, even with the best treatment.
Associated Press writers Fabiola Sanchez in Caracas and Frank Bajak in Bogota, Colombia, contributed to this report.
Ian James on Twitter: http://twitter.com/ianjamesap