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.

"Mr Lee Kuan Yew remains critically ill in the ICU and has deteriorated further," said a statement from the office of his son, Prime Minister Lee Hsien Loong.

On Tuesday, the government said his condition had worsened due to an infection and he was on antibiotics while doctors closely monitored his condition.

In a book published in 2013, the Asian elder statesman said he felt weaker by the day and wanted a quick death.

Prime Minister Lee posted the latest update on his Facebook page and was immediately inundated with expressions of support, prayers and wishes for his father's recovery.

The senior Lee commands immense respect among Singaporeans.

Many expressed their desire to see him live a few months longer to witness the celebrations of the 50th anniversary of Singapore's independence on Aug 9, an event known as "SG50".

It gained self-rule from Britain in 1959 and became a republic in 1965.

"My prayer to Mr Lee Kuan Yew. Hang on. Please be around to celebrate SG50 with the nation," said resident Elena Wee Meng Gek.

Netizen Sangeetha Prasad wrote: "I just don't want him to suffer so much! It is really so heart wrenching! If only love could heal!"

Others were resigned that he may not recover.

"No matter what the outcome is, most important is that Mr Lee Kuan Yew does not suffer. My prayers to him," said Jessie Seah.

AFP - AP