At least 23 people have died in Thailand after a school bus carrying more than 40 students and teachers on a field trip caught fire on the outskirts of the capital Bangkok, police say.

Twenty-three bodies had been identified, forensic science commissioner Trairong Phiwpan said on Tuesday.

An investigation into the causes is under way.

Sixteen students and three teachers were sent to a hospital for treatment, Transport Minister Suriya Juangroongruangkit said.

Images posted on social media and carried by local news outlets showed thick grey smoke pouring out of the bus, parts of which were still on fire.

The blaze was later extinguished.

A Reuters photographer saw fire trucks, police and rescue vehicles parked around the blackened vehicle, with a cluster of firefighters at the entrance.

“Teachers told us that the fire ignited very quickly,” acting police chief Kittirat Phanphet said.

“From speaking to witnesses, we believe the explosion was caused by a spark from the tyre that lit the gas cylinder that was powering the vehicle,” he said, adding that some students escaped through the window.

“We are investigating all individuals, including the bus company to see if this was a case of negligence.”

Suriya said the bus was powered by natural gas.

Prime Minister Paetongtarn Shinawatra said the students were on a field trip from the province of Uthai Thani, about 250 kilometres north of the capital.

“As a mother, I would like to express my deepest condolences to the families,” she said in a social media post on X.