Singapore's founding prime minister, Lee Kuan Yew, was "critically ill" and his condition was worsening while on life support in a hospital, the government said on Wednesday.

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.

The 91-year-old has been receiving treatment for severe pneumonia in the intensive care unit at Singapore General Hospital since Feb 5.