Hugo Chavez's top aides accused the opposition and the media on Thursday of using the Venezuelan president's poor health to wage a "psychological war" to destabilize the country, as the cancer-stricken leader struggles with a severe lung infection.

The stance was adopted after Venezuelan Vice-President Nicolas Maduro returned from Cuba where he visited Chavez, who is suffering from complications more than three weeks after undergoing cancer surgery.

Information Minister Ernesto Villegas said Chavez developed a "severe pulmonary infection" after the surgery led to a respiratory insufficiency, requiring strict adherence to his treatment.

Villegas then leveled the charge that the president's health had become the target of a campaign to destabilize the government.

The government "warns the Venezuelan people about the psychological war that the transnational media complex has unleashed around the health of the chief of state, with the ultimate goal of destabilizing the Bolivarian Republic of Venezuela", Villegas said.

The statement came amid rising demands at home for a detailed account of Chavez's condition and whether he is fit to take the oath of office on Thursday for another six-year term.

Venezuela's constitution calls for new elections to be held within 30 days if the president is unable to take the oath of office or dies during his first four years in office.

But Maduro and National Assembly Speaker Diosdado Cabello, two of the country's top leaders, made clear on their return from Cuba that they were not preparing for a transfer of power.

"Here there is only one transition and it began at least six years ago when it was decreed by Hugo Chavez," Maduro said.

Both men went out of their way to deny rumors of an internal power struggle between them, with Maduro saying they had sworn before Chavez that they would remain united.

"We are here more united than ever," said Maduro, who is Chavez's handpicked successor. "We have sworn before Hugo Chavez, and we reaffirmed to him of our oath ... that we would be united with our people."

Referring to the reported rift, Cabello said the opposition would have to wait "2,000 years for that to happen".

Maduro accused the opposition of "lies and manipulation, a campaign to try to create uncertainty".

Chavez was re-elected on Oct 7 despite his debilitating battle with cancer and the strongest opposition challenge yet to his 14-year leadership in Venezuela, an OPEC member with the world's largest proven oil reserves.

But he has not been seen in public since he underwent a long and complicated surgery 23 days ago for a recurrence of cancer, and officials have acknowledged that his recovery has been difficult.

Cuban doctors first detected the cancer in June 2011, but the Venezuelan government has never revealed what form of the disease he is battling.

Information about his progress has come in comments and micro blog posts by Maduro and a handful of other aides and close allies.

On Thursday, Maduro, who spent five days in Havana, said Chavez was "battling" for his health. "We ask once more for respect for President Chavez's fight to fully regain his health amid a complex situation," he said.

A painting of Venezuelan President Hugo Chavez in Caracas. Top officials returned to the country after gathering in Cuba amid mounting concern about the health of the cancer-stricken president. Raul Arboleda / AFP