"We conveyed our points of concern as their neighbour and said it [Suu Kyi's release] would benefit regional politics," Thaksin Shinawatra said on Thursday after a surprise visit to Pyinmana, the nation's new capital.
Kantathi Suphamongkhon, Thailand's foreign minister who accompanied Shinawatra, said he was "disappointed" that the military government led by Than Shwe would not specify when it would consider releasing the democracy activist.
Suu Kyi, a Nobel prize winner whose National League for Democracy (NLD) party won a 1990 election landslide only to be denied power by Myanmar's army, has been in jail or under house arrest for more than 10 of the past 17 years.
Thaksin said he had spoken on Wednesday to Gloria Arroyo, president of the Philippines and current chair of the Association of South East Asian Nations (Asean), who asked him to convey the association's concern about the lack of democratic reform in Myanmar, formerly known as Burma.
Suu Kyi has been in jail or under
arrest on and off for 17 years
In line with its Asean colleagues, Thailand has a policy of "constructive engagement" with Myanmar's rulers rather than the sanctions and isolation favoured by Europe and the United States.
It is also one of Myanmar's biggest foreign investors and trading partners, particularly in energy and telecommunications.
Myanmar's tightly controlled media made no mention of Thaksin's visit.