SINGAPORE - Prime Minister Lee Hsien Loong said he was saddened to hear about former president S R Nathan's "grave condition".

In a Facebook post on Sunday (July 31) evening, Mr Lee said he was on his way to Washington DC for his official visit to the United States when he received news of Mr Nathan's stroke.

"Have been in touch with his family," Mr Lee wrote. "We have him in our thoughts and prayers."

Mr Nathan, 92, is in critical condition after suffering a stroke on Sunday morning, said the Prime Minister's Office in a statement. He is warded in the Intensive Care Unit at the Singapore General Hospital.

Along with his Facebook post, Mr Lee shared a photo of an antique Littleleaf Japanese Maple tree at the Singapore embassy in Washington, which Mr Nathan planted when he served as ambassador from 1990 to 1996.

Handpicked by Mr Nathan, the tree had been planted as a sapling when the embassy moved into a new chancery building.

His stint as ambassador was among many contributions in a long career, Mr Lee noted.

"He made many good friends for Singapore, and planted many seeds, including this beautiful antique Littleleaf Japanese Maple at our Washington DC embassy," he wrote.

Mr Lee is currently on an official visit to the US, where he will be hosted to a state dinner by US President Barack Obama on Tuesday (US time).