A man arrested with his wife for allegedly murdering their 2-year-old daughter has also admitted to killing four sons born to one of his three former wives, police said on Wednesday.

Songsak Songsaeng, 46, and his wife Sunan Nahuanin, 40, from Kamphaeng Phet, were arrested in Pathum Thani. They were taken to Kamphaeng Phet to show police where they buried their 2-year-old daughter. The girl was found buried under concrete at her mother's house in Kamphaeng Phet.

During questioning at Bang Khen police station, a tearful Mr Songsak admitted to having killed his daughter, though he claimed mental illness drove him to do it, according to police.

He said he felt guilty and wanted to apologise for what he had done.

Police said witnesses told them the family often quarrelled violently.

The couple's arrest came after a social media influencer called on police on Sept 10 to help two girls aged 4 and 12 whom local netizens said were often physically assaulted by their father.

When police arrived at their apartment in Bang Khen district, the girls' pa­rents were not home.

The couple was known to have three children, but there was no sign of the youngest.

Neighbours said the couple had told them they sent the child to live with her grandfather in another province.

After their rescue, the two other girls told police their parents assaulted their youngest sister, causing her death.

A police source said Mr Songsak had 10 children by four women. He had one child each with his first and second wives, five with his third wife and three with his current wife.

The suspect admitted that he had killed four children born to one of his former wives.

The murders occurred in 2013, 2014, 2016 and 2018, according to the police source.