Information Minister Ernesto Villegas gave a brief update on the president, who has been in a military hospital since his return last month from Cuba, where he received medical treatment.
"Today there is a worsening of respiratory function," Villegas announced on Venezuelan state television, adding that Chavez was also receiving "intensive chemotherapy, as well as complementary treatments."
The Venezuelan government has disclosed limited information about Chavez’s illness, fueling uncertainty about whether he will be able to continue in office.
New elections will be held if Chavez dies or is incapacitated.
It was first revealed that Chavez had an unspecified form of cancer in June 2011. Since then, he has undergone at least four operations in Cuba.
After contracting a lung infection, Chavez was forced to remain in Havana for nearly two months while recovering.