CARACAS (REUTERS) - Venezuelan President Hugo Chavez's breathing problems have worsened and he is suffering from a "severe" new respiratory infection as he struggles to recover from cancer surgery, the government said in a medical update on Monday.

The 58-year-old socialist leader has not been seen in public nor heard from in almost three months since undergoing the operation in Cuba. It was his fourth surgery since the disease was detected in mid-2011.

"Today, there is a worsening of his respiratory function, related to his depressed immune system. There is now a new, severe infection," Information Minister Ernesto Villegas said, reading the latest brief statement on Mr Chavez's condition.

Mr Chavez made a surprise pre-dawn homecoming two weeks ago with none of the fanfare and celebration that had accompanied previous returns from treatment in Havana. The government said he is now fighting for his life at a Caracas military hospital.

Armed guards are providing heavy security outside.

"The president has been receiving high-impact chemotherapy, along with other complementary treatments ... his general condition continues to be very delicate," Mr Villegas said.

Mr Chavez suffered multiple complications after the Dec 11 surgery, including unexpected bleeding and an earlier severe respiratory infection that officials said had been controlled. He first underwent surgery in Cuba to treat an abscess in his pelvic area in 2011.

The government said Mr Chavez had trouble speaking because he was breathing through a tracheal tube, but that he was giving orders to ministers by writing them down.

"The commander-president remains clinging to Christ and to life, conscious of the difficulties that he is facing, and complying strictly with the programme designed by his medical team," Mr Villegas said.

Mr Chavez had undergone several grueling rounds of chemotherapy and radiation treatment, which at times left him bald and bloated.

Following an emotional Mass at the military hospital on Friday, Vice-President Nicolas Maduro - Mr Chavez's preferred successor - said the president had decided for himself several days earlier that he would return to Venezuela from Cuba.

Mr Chavez was going to begin a "tougher and more intense" phase of his treatment, Mr Maduro said, and he wanted to be in Caracas.

Opposition leaders have accused Mr Maduro of repeatedly lying about the president's real condition. Several dozen anti-government student protesters have chained themselves up in public to demand proof that Mr Chavez is alive and in Venezuela.

Should the Venezuelan leader step down or die, an election would be held within 30 days and would probably pit Mr Maduro against opposition leader Henrique Capriles, who lost to Mr Chavez in a presidential election in October.

The stakes are also high for the rest of Latin America.

Mr Chavez has been the most vocal critic of Washington in the region and has funded hefty aid programmes for leftist governments from Bolivia to Cuba.