South Korean President Park Geun-hye on Friday expressed her "deep" condolences to Thailand over the death of King Bhumibol Adulyadej, saying his leadership for the Southeast Asian country's harmony will always be remembered.

The king, the world's longest-reigning monarch, died peacefully at a hospital in the Thai capital of Bangkok on Thursday, according to a statement from the country's royal household bureau. He was 88.

Since he took the throne in 1946 at age 18, he had been revered as a rare figure for national unity and stability in the country that has experienced periods of political challenges due to a series of military coups.

"The late (King), who had led the country for 70 years as the father of Thailand and spiritual anchor, has been integral to the country's history, from the birth of the modern state in 1932 to the present day," Park said in a statement.

"His leadership, which has steered the nation towards integration and harmony, will always be remembered," the president added. (Yonhap)