SINGAPORE - Prime Minister Lee Hsien Loong has written to Thai Prime Minister Prayut Chan-o-Cha to express condolences on the death of former Thai premier and president of the Privy Council General Prem Tinsulanonda.

Mr Prem, who was Thailand's prime minister for more than eight years in the 1980s, died of heart failure on May 26 at age 98.

Describing Mr Prem as a "capable and immensely respected leader", Mr Lee said that during Mr Prem's premiership, he steered Thailand through a period of democratic development, holding three elections during his watch and progressively including more elected ministers in each of his five Cabinets.

As the president of the Privy Council from 1998 to 2019, Mr Prem continued to be a loyal and trusted source of advice and counsel to the King and also provided wise counsel to successive military and civilian governments, noted Mr Lee.

Mr Lee said the region also benefited from Mr Prem's leadership, noting that Mr Prem's time as premier coincided with the five countries of Asean coming together decisively to resolutely oppose Vietnam's invasion of Cambodia.

"General Prem was resolute in not accepting this fait accompli. Supported by his able Foreign Minister, Air Chief Marshal Siddhi Savetsila, General Prem worked with Asean partners to support the resistance forces of the Coalition Government of Democratic Kampuchea from Thai territory, and to oppose the Vietnamese occupation in international forums.

"This effective collective resistance prevented a military invasion and regime change from being legitimised, and protected the security of other Southeast Asian countries. Eventually the invasion forces withdrew, a peace settlement was signed, and internationally supervised elections were held to elect a new Cambodian government.

"This decisively shaped the subsequent course of Southeast Asia. It paved the way for Vietnam, Cambodia and Laos to join ASEAN, as partners in promoting the region's peace and development," Mr Lee wrote.

Mr Lee said Mr Prem was also a good friend of Singapore whom the late founding prime minister Mr Lee Kuan Yew had deep respect for as a statesman.

"He strengthened relations between our two countries, including between our militaries, and Singapore and Thailand forged a strong and enduring partnership," Mr Lee said.

Mr Lee said Mr Prem will be greatly missed by the people of Thailand and all who knew him.