Ernesto Villegas told a national television and radio audience the government had "full confidence" in Chavez's medical team in Cuba.
He also said international media outlets were using the 58-year-old leader's illness as part of a "psychological warfare" campaign to "destabilize" the country.
He described Venezuela as having an "ironclad unity" among its government, military and people in their support of "the leadership and political ideals" of Chavez.
"The government of the Bolivarian Republic of Venezuela is complying with its duty to inform the Venezuelan public and our sister nations about the clinical progress of President Hugo Chavez," Villegas said in his 2-minute address late Thursday night.
"Following a delicate surgery Dec. 11, President Chavez has faced complications as a result of a severe respiratory infection," Villegas said. "This infection has led to respiratory deficiency that requires President Chavez to remain in strict compliance with his medical treatment."
He provided no details about the treatment or Chavez's prognosis.
His announcement was the first time the government characterized the infection as severe.
Chavez -- who declared himself fully recovered from the unspecified cancer July 9 -- won his fourth term Oct. 7 and is due to be sworn in for a new six-year term Thursday.
National Assembly President Diosdado Cabello has said the swearing-in ceremony would be postponed if Chavez can't make it. But opposition leaders say postponing the inauguration would be unconstitutional.
The Miami Herald's Spanish-language edition reported a succession plan was being orchestrated in Havana that would have Cabello temporarily assume the presidency while awaiting new elections in which Vice President Nicolas Maduro would be the Chavista candidate.
Preparations for the transition began months ago under the direction of Chavez himself, and final details are now being worked out, a source told El Nuevo Herald.
The communist government of Raul Castro is exerting a great deal of influence in decision making, another source told the newspaper.
"The Cuban sponsors are working behind the scenes to try to create a sort of Politburo, a council that operates in consensus and guarantees the stability of Chavism in Venezuela by bringing together potential heirs and rivals," one of the sources with access to the information in Cuba told the newspaper.
Cuba wants Maduro to succeed Chavez, as Chavez requested before he flew to Havana Dec. 10, the source said.
But Maduro can't be president right away because Venezuela's Constitution requires the National Assembly president to take power if the president-elect can't assume office during a 30-day period before constitutionally required new elections.
So under the Cubans' plan, Cabello would temporarily hold power and Maduro would be the ruling United Socialist Party's candidate, the Herald said.
At the same time, this strategy is of concern in Cuba, the source said, because Cabello has accumulated a lot of power over the years and has few ties to the Cuban revolution.
But despite the reservations, the plan's designers believe Cabello is needed to sustain Chavism in a post-Chavez era, in part because he enjoys great influence over military leaders, the Herald said.