Thailand's king has offered his condolences to the bereaved families of victims in Thursday's gun and knife attack that killed 36 people, including 24 children.

King Maha Vajiralongkorn and Queen Suthida on Friday visited a hospital near the scene of the tragedy that took place at a childcare center in the country's northeastern province of Nongbua Lamphu.

The king told the victims' families that he was deeply saddened that such an incident had happened.

He said there were no words that can express his sorrow, and that he came to offer emotional support.

It is unusual for the king to directly speak to grieving families.

The attacker, a former police officer, went home after the attack, and killed his wife and child before taking his own life.

The man was dismissed from the police earlier this year for drug possession. Police say he had an argument with his wife on the day of the attack.

The man had reportedly been addicted to drugs, although no traces of drugs were detected in his body.