Hugo Chavez has suffered a number of "new complications" following his latest round of cancer surgery, according to officials.
Venezuela's vice-president Nicolas Maduro said the complications came amid a respiratory infection during a televised address from Havana, where Chavez is in hospital.
"Thanks to his physical and spiritual strength, Comandante Chavez is facing this difficult situation."
The Venezuelan leader has not been seen or heard from since undergoing his fourth cancer-related surgery on December 11.
He is due to be inaugurated for a new six-year term on January 10.