Hugo Chavez has suffered "new complications" following his cancer surgery in Cuba and his condition is "delicate", Venezuela's Vice President Nicolas Maduro has said.
In a televised address from Havana, Mr Maduro said: "We have been informed of new complications that arose as a consequence of the respiratory infection we already knew about.
"President Chavez's condition is still delicate," Mr Maduro added, warning that the unspecified complications "are being treated in a process that is not without its risks".
Mr Maduro, who said he had spoken with Mr Chavez, added that the 58-year-old president was facing a "tough situation".
The vice president said he would stay in Havana in the coming hours "with El Comandante and his family, closely following his condition and its evolution".
"We trust that the global outpouring of love for Chavez will help him heal", said Mr Maduro, with Rosa Virgina Chavez, one of the president's daughters, Science and Technology Minister Jorge Arreaza, and top prosecutor Cilia Flores.
The vice president gave his address alongside Mr Chavez's eldest daughter, Rosa, and son-in-law Jorge Arreaza, as well as Attorney General Cilia Flores.
A supporter of Mr Chavez writes a message for him in Caracas The Venezuelan leader, who has been the face of the Latin American left for more than a decade, has not been seen or heard from since undergoing his fourth cancer-related surgery on December 11.
Mr Chavez, who has been in power since 1999, won another six-year term in October's presidential election.
He is scheduled to be sworn in on January 10, but his health has raised concerns over the future of his leftist movement and whether he will even be well enough to attend the inauguration.
On Monday, the Venezuelan government said there had been a "slight improvement" in his condition as he recovers from the post-operative respiratory infection.
Officials have never disclosed the type or severity of Mr Chavez's cancer, first diagnosed in June 2011, and he only designated a political successor - Mr Maduro - earlier this month.
The Venezuelan leader had, in fact, asserted before embarking on his arduous re-election campaign earlier this year that he was cancer-free - but he was later forced to admit he had suffered a relapse.
Afghan officials say the couple were kidnapped in Wardak Province (above)
Nancy Lanza was killed at the home she shared with her son
Last year's London celebrations saw Big Ben lit up
The passenger plane was sheared in two
Vnukovo is Russia's third biggest airport
Mourner Meena Rai (L) says the victim was planning to get married
Thousands take part in a candlelight vigil for the victim in New Delhi
Police had braced for violent protests, but most passed of peacefully
The horrific crime has sparked an angry debate over women's safety in India
The 1996 massacre in Tasmania sparked anti-gun protests
The guesthouse frrom where Martin Bryant killed 35 people
The Australian government bought back guns and destroyed them
The victim's body has arrived back in India from Singapore
Indian police are braced for more protests
Activists light candles in memory of the rape victim
Police have closed-off parts of New Delhi with metal barriers
House Speaker John Boehner arrives at the White House for the talks
Gen. Schwarzkopf was known as Stormin' Norman for his explosive temper
The General salutes an Iraqi general to set Gulf War cease-fire terms
The General receiving the medal of Grand Officer of the Legion of Honour
The ex-Italian PM attended the 18th birthday party of Noemi Letizia
Mr Berlusconi has confirmed he is dating Francesca Pascale
Netflix allows customers to access films online
The storm has blanketed the Midwest
Snow and rain is expected to hit New York City
Issue 700 is the last of The Amazing Spider-Man series 
Victims: Michael Chapperini (left) and Tomasz Kaczowka
Webster is northwest of New York City
The high-speed passenger service is under way
The line runs from Beijing in the north to Guangzhou in the south
Storm clouds head towards the East Coast
A man is put in an ambulance after a tornado in Mobile
Lanza used a Bushmaster rifle, similar to this one from Rock River Arms
Gun control advocates also want high-capacity magazines banned
Officials say that at the moment there is no need to evacuate
The adviser died at the Kabul police headquarters in the Afghan capital
King Abdullah has pushed for reforms to the country's legal system
Water cannon was used on crowds in New Delhi
The rape of a young woman has sparked a public outcry
Life is still tough for many in the former South Africa township
The charity gives people a chance of a better life through music
A woman shovels snow from a sidewalk in Baraboo, Wisconsin