SINGAPORE, March 19 -- Singapore's former Prime Minister Lee Kuan Yew still remained critically ill in the intensive care unit (ICU), the Prime Minister's Office (PMO) said on Thursday.

Lee's worsening condition was announced on Tuesday, when the PMO said he had an infection and was being treated with antibiotics. Doctors were monitoring his condition.

Lee, 91, has been in hospital since Feb. 5 with severe pneumonia. The PMO said on Feb. 28 that his condition had improved slightly but remained on antibiotics. It also said the founding leader continued to be sedated and on mechanical ventilation.

Lee co-founded the ruling People's Action Party and became the premier of the newly-born republic in 1965. He stepped down as prime minister in 1990.