The health of Lee, who has been on a ventilator, "has worsened due to an infection," the government said in a brief statement. "Doctors are closely monitoring his condition."
Lee is credited with transforming the backwater city-state of Singapore into one of the world's most efficient and prosperous international business centers, though a place that shows little tolerance for political dissent.
Lee was admitted to Singapore General Hospital on Feb. 5 for what authorities described as "severe pneumonia." In late February, the prime minister's office said his condition had improved slightly, but that he remained sedated and on a mechanical ventilator.
Lee became Singapore's prime minister in 1959 and resigned in 1990, later taking on special titles including senior minister and "minister mentor." His son, Lee Hsien Loong, became prime minister in 2004.