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
 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.
 Nancy Lanza was killed at the home she shared with her son
 Nancy Lanza was killed at the home she shared with her son   Last year's London celebrations saw Big Ben lit up
 Last year's London celebrations saw Big Ben lit up   Sydney's fireworks are expected to be bigger than last year
 Sydney's fireworks are expected to be bigger than last year   The passenger plane was sheared in two
 The passenger plane was sheared in two   Vnukovo is Russia's third biggest airport
 Vnukovo is Russia's third biggest airport   Mourner Meena Rai (L) says the victim was planning to get married
 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
 Thousands take part in a candlelight vigil for the victim in New Delhi   Police had braced for violent protests, but most passed of peacefully
 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 horrific crime has sparked an angry debate over women's safety in India   The 1996 massacre in Tasmania sparked anti-gun protests
 The 1996 massacre in Tasmania sparked anti-gun protests   The guesthouse frrom where Martin Bryant killed 35 people
 The guesthouse frrom where Martin Bryant killed 35 people   The Australian government bought back guns and destroyed them
 The Australian government bought back guns and destroyed them   The victim's body has arrived back in India from Singapore
 The victim's body has arrived back in India from Singapore   Indian police are braced for more protests
 Indian police are braced for more protests   Activists light candles in memory of the rape victim
 Activists light candles in memory of the rape victim   Police have closed-off parts of New Delhi with metal barriers
 Police have closed-off parts of New Delhi with metal barriers   Gen. Schwarzkopf was known as Stormin' Norman for his explosive temper
 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 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 General receiving the medal of Grand Officer of the Legion of Honour   The storm has blanketed the Midwest
 The storm has blanketed the Midwest   Snow and rain is expected to hit New York City
 Snow and rain is expected to hit New York City   Netflix allows customers to access films online
 Netflix allows customers to access films online  
 Victims: Michael Chapperini (left) and Tomasz Kaczowka
 Victims: Michael Chapperini (left) and Tomasz Kaczowka   Webster is northwest of New York City
 Webster is northwest of New York City   Storm clouds head towards the East Coast
 Storm clouds head towards the East Coast   A man is put in an ambulance after a tornado in Mobile
 A man is put in an ambulance after a tornado in Mobile   The adviser died at the Kabul police headquarters in the Afghan capital
 The adviser died at the Kabul police headquarters in the Afghan capital   King Abdullah has pushed for reforms to the country's legal system
 King Abdullah has pushed for reforms to the country's legal system   Water cannon was used on crowds in New Delhi
 Water cannon was used on crowds in New Delhi   The rape of a young woman has sparked a public outcry
 The rape of a young woman has sparked a public outcry   Life is still tough for many in the former South Africa township
 Life is still tough for many in the former South Africa township   The charity gives people a chance of a better life through music
 The charity gives people a chance of a better life through music   The clashes Friday in Alexandria were the latest in four weeks of turmoil
 The clashes Friday in Alexandria were the latest in four weeks of turmoil   Sandy Hook Elementary has been closed indefinitely
 Sandy Hook Elementary has been closed indefinitely   Support has grown for a ban on assault weapons
 Support has grown for a ban on assault weapons   The tiny French village of Bugarach drew global attention
 The tiny French village of Bugarach drew global attention   The end is nigh! How some were counting down the seconds
 The end is nigh! How some were counting down the seconds   A man in China made tsunami-proof survival pods
 A man in China made tsunami-proof survival pods   One of the children hurt in Friday's attack talks to her mum on the phone
 One of the children hurt in Friday's attack talks to her mum on the phone   Students stand behind a policeman as they learn to defend themselves
 Students stand behind a policeman as they learn to defend themselves   Mr Modi addresses his supporters during an election rally in Dokar
 Mr Modi addresses his supporters during an election rally in Dokar   Protesters demonstrated outside the NRA's headquarters in Washington DC
 Protesters demonstrated outside the NRA's headquarters in Washington DC   Adam Lanza killed his mother before shooting dead 20 children and six staff
 Adam Lanza killed his mother before shooting dead 20 children and six staff  
 Christopher Steven's murder was the first of a US ambassador since 1988
 Christopher Steven's murder was the first of a US ambassador since 1988  

