Hugo Chavez, 58, had his fourth operation for cancer in Cuba on December 11 and then developed a respiratory infection.
In a statement from Caracas, Information Minister Ernesto Villegas said the infection had “led to a respiratory insufficiency”.
Hugo Chavez is due to be sworn in for another term in office on January 10.
“[Mr.] Chavez has faced complications as a consequence of a severe lung infection,” said Ernesto Villegas.
The president has not been seen in public since having cancer surgery in Havana, prompting persistent speculation about his condition.
The information minister was among a number of Hugo Chavez’s political allies who have urged people not to believe what they called lies about his health.
Ernesto Villegas warned Venezuelans not to be taken in by a “psychological war” being fought in the media which had the “ultimate aim of destabilizing the Bolivarian republic”.
Vice-President Nicolas Maduro and National Assembly Head Diosdado Cabello visited Hugo Chavez this week, along with several other dignitaries.
Nicolas Maduro said on Wednesday that President Hugo Chavez was “conscious” and had gripped his hand firmly as they discussed Venezuelan politics.
He and Diosdado Cabello flew back to Caracas on Thursday and dismissed rumors of a split in the governing socialist movement.
“We’re more unified than ever,” Vice-President Nicolas Maduro said upon his return to Venezuela from Cuba.
“We swore in front of Commander Chavez that we’ll be united at the side of our people,” he said.
Opposition activists have speculated about a split between Nicolas Maduro, whom President Hugo Chavez named as his preferred successor, and Diosdado Cabello, who the constitution decrees should temporarily take over power if the Venezuelan leader were to have “an absolute absence”.
Diosdado Cabello said that those who were spreading rumours about problems between him and Nicolas Maduro would “have to wait 2,000 years for that to happen”.
Hugo Chavez has been in power since 1999 and was elected for a fourth term in office in October.
It is unclear if he will be able to attend the swearing-in ceremony for another six-year term in office next week, nor what the procedure would be if he was unable to attend.
Diosdado Cabello recently said that the ceremony could be delayed. But the opposition says such a move would be unconstitutional.
Opposition politicians say there have been no independent reports of the president’s condition.
On Thursday, Caracas Mayor Antonio Ledezma demanded that a commission travel to Cuba to report back on Hugo Chavez’s health.
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