Former president S R Nathan, 92, who had a stroke on Sunday, remains critically ill and unconscious, but his vital signs are stable, said Prime Minister Lee Hsien Loong.

Mr Lee had gone to the Singapore General Hospital (SGH) yesterday to see Mr Nathan, hours after returning to Singapore from his official visit to the United States.

He said in a Facebook post that Mr Nathan "remains critically ill and unconscious, but he is restful and his vital signs are currently stable".

He added: "Not many people have done as much for Singapore as Mr Nathan has. Please continue to keep him in your thoughts and prayers."

Mr Nathan, Singapore's sixth and longest-serving president, had a stroke on Sunday morning and was admitted to the hospital's intensive care unit. Since then, friends, including current and former Cabinet ministers and senior civil servants, have been visiting him in hospital.

Mr Lee was then on his way to the US for an official visit.

Minister in the Prime Minister's Office Chan Chun Sing, who was part of Mr Lee's delegation, also visited Mr Nathan yesterday.

Mr Lee, who met Mr Nathan's wife and family at SGH yesterday, said: "This is a difficult time for them.

"The support and encouragement they have received from so many people means a lot to them. They have been touched by the many messages and good wishes."

Mr Lee and Mr Chan also thanked Mr Nathan's team of doctors and nurses for their attentive care.

Mr Nathan, who was president from 1999 to 2011, suffered a stroke in April last year and had to be hospitalised for almost three weeks.