Singapore will hand the Chairmanship of ASEAN to Thailand in July this year, and PM Lee believes that a 'cohesive' ASEAN will be built.

'Your appointment marks a significant milestone in Thailand's recent political evolution. I am confident that Thailand will continue to progress and develop under your leadership,' PM Lee wrote.

Mr Samak Sundaravej was chosen by Thailand's Parliament on Monday to be the country's new prime minister, after the People's Power Party won the largest number of seats in elections last month.

'I look forward to working with you to further deepen our bilateral relations, and to advance our regional cooperation and integration efforts,' said PM Lee in the letter.