Thailand's prime minister on Wednesday vowed "preventive measures" after a shooting at a Bangkok shopping mall left two people dead and raised fresh questions about the kingdom's gun control.

Shoppers returned in dribs and drabs as the Siam Paragon mall reopened less than 24 hours after the shooting – Thailand's third high-profile deadly gun attack in four years.

The shooting at one of Bangkok's biggest, most upmarket malls will come as a fresh blow to Thailand's efforts to rebuild its vital tourism industry after the pandemic.

Prime Minister Srettha Thavisin joined a minute's silence at the mall before offering the government's condolences to the families of the two female victims – one Chinese and one from Myanmar.

"I am confident Siam Paragon and government officials did their best to minimise the casualties and damage," he said.

"Let this be the only time this happens. My government insists we will give priority to preventive measures," he added, without giving details.

The victim from Myanmar was named by her country's foreign ministry as 31-year-old Moe Myint.

Police have charged a 14-year-old suspect with premeditated murder, attempted murder, carrying and firing a gun in a public place and owning an unlicensed firearm.

Police Major General Nakarin Sukontawit said the boy, a student at a US$4,000-a-term private school just metres from Siam Paragon, was undergoing psychiatric tests to see if he would be fit to stand trial.

Investigators said on Tuesday the boy had been undergoing treatment for mental illness but had stopped taking his medication and reported hearing voices telling him to shoot people.

UNICEF Thailand deputy representative Severine Leonardi said the incident should accelerate efforts to improve the kingdom's youth mental health services.

Samran Nuanma, Assistant National Police Chief, told a news conference on Wednesday that the weapon used in the attack was a blank-firing pistol.

"But the barrel was modified for live shooting," Samran said.

"We will increase regulations and laws to control the use of firearms." (AFP)