"During this Christmas season... I have had to fight for my health again, in order to continue devoting myself completely for the happiness of Venezuela," Chavez said in a message read by Vice President Nicolas Maduro.

He thanked the "commitment and loyalty" of his supporters "in this very difficult time."

Maduro announced that he was heading a government delegation to Cuba to visit Chavez for the first time since the December 11 surgery, saying Electricity Minister Hector Navarro would assume the vice presidency during his absence.

Maduro did not specify how long the group would stay in Havana.

Chavez, 58, the face of the Latin American left for more than a decade and a firebrand critic of US "imperialism," has been in power since 1999.

He won another six-year term in October's presidential election, and 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 government said there had been a "slight improvement" in his condition as he recovers from a post-operative respiratory infection.

Officials have never disclosed the type or severity of Chavez's cancer, which was first diagnosed in June 2011, and he only designated a political successor -- 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.