Venezuelan President Hugo Chavez is still suffering a "severe" respiratory infection that has hindered his breathing as he struggles to recover from cancer surgery in Cuba.

The 58-year-old socialist leader has not been seen in public nor heard from in more than three weeks.

Officials say he is in delicate condition after his fourth operation in just 18 months for an undisclosed form of cancer in his pelvic area.

"Comandante Chavez has faced complications as a result of a severe lung infection," information minister Ernesto Villegas said in the latest official update on the president's condition.

"This infection has caused a breathing insufficiency that requires Comandante Chavez to comply strictly with medical treatment," he added, giving no further details.

Vice President Nicolas Maduro had earlier returned to Venezuela after visiting Mr Chavez in hospital amid rumours that the president could be close to death.

Flanked by senior government figures including Diosdado Cabello, the head of the National Assembly, Mr Maduro toured a coffee production plant in Caracas - the type of visit that the president made frequently before he fell ill.

"He is conscious of the battle that he's in, and has the same fighting spirit as always, with the same strength and energy as always, with his confidence and security," Mr Maduro said.

"We're going to be alongside him with the same strength and the same energy."

Mr Maduro said Mr Cabello, oil minister Rafael Ramirez and Mr Chavez's elder brother Adan, among others, had all been with the president in the Havana hospital.

Venezuelan bonds rallied to five-year highs earlier on rumours that Mr Chavez's health had taken a turn for the worse.

Foreign investors generally hope for a more business-friendly government in Venezuela, and its assets have rallied in recent months on news of his illness.

Mr Chavez is still set to be sworn-in again as president on 10 January, as spelled out in the constitution.

If he were to die or had to step aside, new elections would be held within 30 days, with Mr Maduro running as the ruling socialist party (PSUV) candidate.

While the constitution gives 10 January as the start of a new presidential term, it does not explicitly state what happens if a president-elect cannot take office on that date.

Top PSUV officials have suggested that Mr Chavez's inauguration could be postponed, while the opposition says any delay would be just the latest sign the former soldier is not fit to govern.