"They're not telling us anything about his 'fundamental illness,'" Castro said. "They don't touch that subject. So, the question is: What's happened with the cancer?"
In a downtown Caracas plaza, some cheered and clapped Friday as they watched the government broadcast replayed on a television under a tent where the president's supporters regularly gather.
"I have prayed like you wouldn't believe for the health of our commander president," gushed Luisa Rodriguez, saying the pictures filled her with joy.
People bought photos of Chavez that were being sold in the square for the equivalent of $3 apiece.
Before leaving for Cuba, Chavez acknowledged there were risks and said that if his illness prevented him from staying on as president, Vice President Nicolas Maduro should run in a new election to take his place.
Maduro has traveled repeatedly to Havana in recent weeks and has shown documents he said were signed by Chavez while insisting the president remains in charge. On Wednesday, Maduro said Chavez is undergoing "extremely complex and tough" treatments, which he didn't specify.
Arreaza on Friday described them as "palliative treatments" but didn't give details. He said Chavez has been keeping up his spirits with "llanera" folk music playing in his room.
"He's a fighter," Arreaza said in a televised interview.
The 58-year-old president has been undergoing cancer treatment in Cuba on-and-off since June 2011. He has said he has had tumors removed from his pelvic region and has undergone chemotherapy and radiation treatment.
Throughout the treatments, Chavez has not revealed the type of cancer or the location where tumors have been removed.
The government provided the update a day after students began protesting outside the Cuban Embassy demanding the president appear and accusing the island's leaders of wielding undue influence in Venezuela's affairs.
Opposition leader Henrique Capriles said he hopes the president recovers, but also accused the government of misleading people about the illness.
"There are some government leaders who weren't elected and who appear every day on television with a different story," Capriles told reporters. In a message on Twitter, he said that "several days ago the liars said they spoke with him. ... Now, they say he can't speak. They're deceiving their own people!"
Associated Press journalists Fabiola Sanchez, Christopher Toothaker and Ricardo Nunes in Caracas, Frank Bajak in Lima, Peru, and Cesar Garcia in Bogota, Colombia, contributed to this report.
Ian James on Twitter: http://twitter.com/ianjamesap