Lee was admitted to Singapore General Hospital on Feb. 5 for severe pneumonia and was later put on life support.
Lee's "condition has worsened due to an infection. He is on antibiotics. The doctors are closely monitoring his condition," the Prime Minister's Office said in a statement posted on its website.
Singapore's first prime minister, Lee ruled for 31 years until 1990, and has been credited with transforming the city-state from a sleepy tropical port to a wealthy, bustling financial hub with one of the highest average incomes in the world.
Under Lee and his successors, including his son, current Prime Minister Lee Hsien Loong, Singapore — known for its ban on chewing gum sales and canings for crimes some countries would rule as minor — has strictly controlled public speech and assembly, though it has become socially more liberal and allowed greater artistic freedom in recent years.
Lee commands immense respect among Singaporeans, who this year will celebrate the country's 50th independence anniversary.