SINGAPORE - President Halimah Yacob has paid tribute to former Vietnamese president Le Duc Anh for his contributions to Singapore-Vietnam relations.

In her letter to her Vietnamese counterpart Nguyen Phu Trong on Friday (April 26) to convey her condolences on the passing of Anh, President Halimah said the former leader played an important role in deepening bilateral relations between Singapore and Vietnam.

"His Presidency took place at a time when relations between our two countries were growing rapidly. This included the establishment of the first Vietnam-Singapore Industrial Park in 1996, which has now become a flagship of our bilateral ties," she said.

"Singapore has lost a close friend. Please accept my sincere and deepest condolences during this time of sorrow."

Anh, 99, died late on Monday "following a long illness", the government and state media announced a day later.

Born in 1920, the former general led the invasion of Cambodia which led to the fall of the Khmer Rouge regime.

"In his glorious military career, General Le Duc Anh fought for more than 30 years in the resistance wars against the French, the Americans, and 10 years in the war (in Cambodia)," Tuoi Tre newspaper reported.

Anh served as president between 1992-1997, championing the continued primacy of the Communist Party even as Vietnam embarked on sweeping market reforms that spurred remarkable economic growth.

He also played "an important role in normalising diplomatic relations" between Vietnam and the United States, and in improving relations with China, the state-run Viet Nam News reported.

During his tenure, US president Bill Clinton formally established diplomatic ties with the government in Hanoi in 1995.

In that same year, he became the first Vietnamese head of state to set foot on US soil after the Vietnam War when he attended the 50th anniversary of the United Nations in New York.

Anh stepped down as president in 1997, a year after suffering what news reports described as a major stroke.