Venezuelan Vice President Nicolas Maduro says President Hugo Chavez is "battling for his health, his life."
Maduro said on state television on February 28 that Chavez's condition was "complex and difficult," but gave no further details.
The vice president said Chavez's health was failing because he dedicated his whole life to his people.
"You all know why he didn't take care of his health, because he gave his body and soul and forgot about his obligations to himself so he could give everything to the people: work, life, housing, food, education, health care," Maduro said. "That's the real truth."
Chavez returned to Venezuela on February 18 after spending two months in Cuba undergoing treatment for cancer.
Venezuelan officials have never said what type of cancer Chavez was treated for or why it was necessary to travel to Cuba.
The 58-year-old Chavez has undergone four operations, chemotherapy, and radiation treatment since June 2011, when he first announced he had cancer.
He has not been seen publicly since he traveled to Cuba for his latest surgery in December.
On February 15, the government released pictures of Chavez with his two daughters at his bedside -- the only public images of the president in two months.
The government last week said Chavez was breathing with the help of a tracheal tube following a respiratory infection.
Chavez was reelected for another six-year term as president in October 2012, but he was too ill to return from Cuba for his inauguration in January.
Chavez has said that if he was unable to return to power, Maduro was his preferred successor.
Under the constitution, an election must be called within 30 days if the president becomes incapacitated.
Chavez has led OPEC-member Venezuela for 14 years and has built good relations with U.S. adversaries such as Iran and Syria.
His supporters say Chavez speaks for the poor, while his critics have accused him of becoming increasingly autocratic.
With reporting by AFP and BBC