Hugo Chávez has been receiving chemotherapy since recovering from a severe respiratory infection in mid-January and "continues his battle for life", his vice-president has said.

It is the first time anyone in the Venezuelan government has mentioned chemotherapy as a treatment being received by the president since he had cancer surgery in Cuba on 11 December.

Nicolás Maduro made the disclosure after a mass for Chávez in a new chapel outside the military hospital where authorities say the president has been since being flown back to Caracas on 18 February.

The vice-president quoted Chávez as saying he had decided to return toVenezuela because he was entering "a new phase" of "more intense and tough" treatments and wanted to be in Caracas for them.

Maduro's comments came hours after an accusation by the opposition leader, Henrique Capriles, that the government had lied about Chávez's condition.

"We'll see how they explain to the country in the [coming] days all the lies they have been telling about the president's situation," Capriles, whom Chávez defeated in elections on 7 October, said in a tweet.

Chávez has not been seen or heard from since going to Cuba for his fourth cancer surgery, except for a set of "proof of life" photos released on 15 February while he was still in Havana.

Chávez first revealed an unspecified cancer in the pelvic region in June 2011, and reported undergoing radiation treatment and chemotherapy after earlier operations.

The government has sent mixed signals on Chávez's condition, although Maduro has said several times that he is battling for his life. He repeated that on Friday, and also accused opponents of spreading rumours about Chávez's health to destabilise the nation.

Maduro, Chávez's chosen successor, said his boss's condition was extremely delicate over the new year as he battled a respiratory infection that required a tracheal tube.

"In mid-January he was improving, the infection could be controlled, but he continued with problems of respiratory insufficiency. Afterwards, there was a general improvement, and the doctors along with President Chávez decided to initiate complementary treatments," Maduro said.

"You know what the complementary treatments are, right? They are chemotherapy that is applied to patients after operations."

The opposition says Chávez should either be sworn in for the new term he won in the election or declare himself incapable and call a new election. The constitution says he should have been sworn in on 10 January, but Venezuela's supreme court said it could be delayed.