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.
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