Nicolas Maduro, Venezuela's vice president, has said President Hugo Chavez is suffering from "new complications" following his cancer surgery in Cuba.
Maduro did not give details on Sunday about the particular issues, which he said came amid a respiratory infection. He said Chavez's condition remains "delicate".
Maduro, who said he had spoken with Chavez about his health complications and national affairs, added that the president was facing a "tough situation".
"Comandante Chavez particularly wanted us to relay his New Year's greetings to every Venezuelan family who are gathered together across the country at this time," Maduro said.
Maduro had arrived in Havana on Saturday in a sudden and unexpected trip to visit Chavez. He said on Sunday that he would remain in Havana "for the coming hours" but did not specify how long.
Officials have said that Chavez already suffered unexpected bleeding caused by the six-hour operation for an undisclosed form of cancer in his pelvic area.
Medical experts say that it is common for patients who have undergone major surgeries to suffer respiratory infections and that how a patient fares can vary widely from a quick recovery in a couple of days to a fight for life on a respirator.
His resignation for health reasons, or his death, would upend the politics of the South American OPEC nation where his personalised brand of oil-financed socialism has made him a hero to the poor but a pariah to critics.
'Lot of uncertainty'
Al Jazeera's Teresa Bo, reporting from Caracas, said the country "is suddenly left with a lot of uncertainty" after Maduro left for Cuba.
"The president's supporters are extremely worried about the current situation," Bo said. "They are afraid that political instability could take over Venezuela once again."
A planned New Year celebration in Caracas was also cancelled at the last minute, Bo reported.
Maduro's latest update differed markedly from last Monday, when he had said he received a phone call from the president and that Chavez was up and walking.
The Venezuelan leader has not been seen or heard from since undergoing his fourth cancer-related surgery on December 11, and government officials have said he might not return in time for his scheduled January 10 inauguration for a new six-year term.
According to the Venezuelan constititution, if Chavez is not reinaugrated by the January 10, as is scheduled, then the president of congress will take over and call for new elections in 30 days.
However, Diosdado Cabello, President of Venezuela's National Assembly, has said that inaugration ceremony will be postponed if Chavez will not attend it.
Before Chavez left for Cuba, he acknowledged the precariousness of his situation and designated Maduro as his successor, telling supporters they should vote for the vice president if a new presidential election was necessary.
"There would be a big sympathy vote for the Chavez-chosen candidate," said Colin Harding, Latin American Affairs Specialist. He added that Maduro would be assured a solid vote.
Harding told Al Jazeera that he did not expect the relationship with the US to improve at all under Maduro. "It is possible that he would be less confrontational than Chavez."
"Once Chavez is off the scene, almost everything changes in Venezuela and it is extremely hard to predict what might happen."