Police in Thailand say they have arrested three men on suspicion of illegally selling a modified blank gun and ammunition used by a teenager in a deadly shooting at a shopping mall in Bangkok.

Police said on Thursday they had taken into custody two men in southern Thailand and another in Bangkok. The shooting two days earlier left a Chinese national and a Myanmar woman dead, and five others wounded.

Police say the 14-year-old suspect is believed to have used social media to buy a blank gun that had been modified to fire real bullets. Police are trying to determine the motive for the attack.

The government responded to the shooting by expressing its intention to tighten gun control.

More than 10 million privately owned guns were estimated to be in circulation in Thailand in 2017, the most of any Southeast Asian country.

In October last year, a former police officer went on a rampage at a childcare center in northeastern Thailand using a gun and a knife. He killed 36 people, including 24 children.