Prime Minister Lee Hsien Loong stressed Singapore's warm and friendly relations with Sri Lanka as he extended congratulations to the South Asian nation's new premier yesterday.

In his letter to Mr Mahinda Rajapaksa on his election victory, Mr Lee said Singapore and Sri Lanka's relationship is underpinned by long-standing economic and people-to-people ties between the two states.

"As our countries commemorate 50 years of diplomatic relations this year, I look forward to working with you to strengthen our cooperation, and to lay the groundwork for the next 50 years of our relationship," Mr Lee wrote.

He wished Mr Rajapaksa good health and Sri Lanka success in the fight against the coronavirus.

Mr Rajapaksa was sworn in as Sri Lanka's new Prime Minister on Sunday, after his party secured a landslide victory in the Aug 5 parliamentary election.

His party won 145 seats in the vote to elect new legislators in a 225-member Parliament.

The new Parliament will convene on Aug 20, according to Mr Rajapaksa's brother, Sri Lankan President Gotabaya Rajapaksa.

The ballot was one of the most peaceful in Sri Lankan history, with a 71 per cent voter turnout, according to a Reuters report citing the Elections Commission.