SINGAPORE - Singapore Prime Minister Lee Hsien Loong on Friday wrote a letter of condolence to his Thai counterpart Prayut Chan-o-cha over a mass killing that claimed the lives of at least 38 people in Thailand's north-east, the Ministry of Foreign Affairs said in a statement.

Those killed included 23 children at a daycare centre.

In his letter, PM Lee said he was shocked and saddened by the incident that took place in Nong Bua Lamphu province on Thursday.

"This senseless act of violence took many innocent lives, most of whom were children," added PM Lee.

"On behalf of the Singapore Government, I offer my deepest condolences to the bereaved families of the victims in this heartbreaking tragedy," he said, adding that he wished for all the injured to make swift recoveries.

PM Lee also said this "heinous act deserves to be condemned in the strongest terms".

"Singapore stands in solidarity with Thailand during this difficult time."

A former policeman burst into the daycare centre in a knife and gun rampage on Thursday.

Most of the children who died at the facility in Uthai Sawan, a town 500km north-east of Bangkok, were stabbed to death, police said.

It marked one of the worst child death tolls in a massacre by a single killer in recent history.