Chavez said in a speech Wednesday night that Castro's situation has been "delicate" recently, but he said recent Spanish press reports portraying Castro as near death were speculation. He did not elaborate on specifics of the Cuban leader's health.
"About 10 days ago, he called me and we spoke for about half an hour," Chavez said.
Chavez said he was breaking a promise because Castro had allegedly said to him: "I beg you please don't tell anyone that I called you because then they get jealous, and others call me from I don't know where and want to speak with me, but I'm hardly calling anyone."
Chavez gave no further details on their conversation or on Castro's progress, other than saying: "Fidel has been in a delicate situation, and he himself said it: this will be a slow process and not free from risks, (and) it has been a serious complication."
The 80-year-old Castro has not been seen in public since he underwent intestinal surgery in July. He temporarily ceded his powers to his 75-year-old brother Raul.
His medical condition has been kept a state secret, and Cuban officials have insisted he is recovering.
Chavez said Castro "is one of those men who will never die. He will always live on."
Spanish newspaper El Pais has reported that Castro is suffering from diverticulitis, an inflammation of the colon that can lead to serious infection if contents from the intestine leak into the abdomen, reports AP.
Citing two unidentified sources at the Madrid hospital of surgeon Jose Luis Garcia Sabrido, who flew to Cuba in December to treat Castro, El Pais said Castro had undergone three unsuccessful operations including one in which his entire large intestine removed suggesting he was suffering from a severe infection.
Garcia Sabrido, the hospital's chief surgeon, declined comment Wednesday but said in an interview posted on CNN's Web site that El Pais' account of Castro's condition being grave was wrong.
A Cuban diplomat in Madrid said Tuesday that the newspaper's report was "an invented story."