Pro-Chavez militant Enrique Barroso sounded grave when reached by telephone.
"This is not easy for him nor for us," he said. "We call on the people to pray and hold vigil for the health of the president."
One of Chavez's three daughters, Maria Gabriela, expressed thanks to well-wishers via her Twitter account. "We will prevail!" she wrote, echoing a favorite phrase of her father. "With God always."
There has been speculation that Chavez's cancer has spread to his lungs and can't be halted.
An oncologist not involved in Chavez's treatment, which has been conducted in tightly enforced secrecy, told The Associated Press that he viewed Villegas' statement as recognition that Chavez's condition is "truly precarious."
He called into question the veracity of Villegas' statement that Chavez had been undergoing chemotherapy, saying patients in such a delicate state are not put on chemotherapy.
Maduro said last week, in the first such announcement, that the president had begun receiving chemotherapy around the end of January.
Doctors have said that such therapy was not necessarily to try to beat Chavez's cancer into remission but could have been palliative, to extend Chavez's life and ease his suffering.
Dr. Carlos Castro, scientific director of the Colombian League Against Cancer in Bogota, Colombia, said "it's difficult to predict" when Chavez might die, but he believes "it's a matter of days."
Castro said that Chavez could face further respiratory complications if he receives more intense chemotherapy treatment.
If the president's medical team "gives him strong chemotherapy again, then it would not be surprising if some infections reappear," Castro said in a telephone interview.
While in Cuba, Chavez suffered severe respiratory infection in late December that nearly killed him, Maduro said last week. A tracheal tube was inserted then and government officials have said his breathing remained labored.
They have sent mixed signals throughout Chavez's post-operative days, and in an early February opinion survey nearly three in five Venezuelans said they believed the president whose largesse has endeared him to the poor would recover.
In Cuba, Chavez has undergone a series of radiation treatments and chemotherapy after his operations.
But the entire treatment regimen was kept far from public scrutiny.