Thai police say they are investigating whether a school bus fire that killed 23 young students and teachers in suburban Bangkok was caused by negligence after filing initial charges against the driver.

The fire on the bus carrying six teachers and 39 primary and junior high school students on Tuesday spread so quickly many were unable to escape.

The driver, Saman Chanput, was arrested several hours later and charged with reckless driving causing deaths and injuries, failing to stop to help others and failing to report the accident, police said.

Acting police chief Kitrat Phanphet said authorities were investigating if the fire might be caused by negligence from both the driver and the bus company, and will press charges against all parties responsible.

While an initial investigation suggested that the driver was not speeding, police found 11 natural gas canisters inside the bus that had a permit to install only six, Kitrat said.

Police have not officially concluded the cause of the fire, but they said the driver told investigators he was driving normally until the bus lost balance at its front tire, hit another car and scraped a concrete highway barrier.

The sparks from the friction might have caught on the highly flammable gas canisters and ignited the blaze, police said.

Kitrat said the fact that the driver did not immediately stop after feeling the bus was losing balance could be grounds for negligence.

The inspection of the bus found that its emergency exit could be opened, but it wasn't clear if it worked properly, said chief of police forensics Trairong Phiwpan. He also said they did not find any window breakers.

In an interview with public broadcaster Thai PBS, bus company owner Songwit Chinnaboot said the vehicle was inspected for safety twice a year as required and that the gas cylinders had passed safety standards.

The families of the victims were driven from Uthai Thani, the northern province where the bus departed from on a school trip, to Bangkok to provide DNA samples for the identification process.

Three students were hospitalised, two of them in serious condition.

White-and-gold coffins carrying the bodies were loaded onto ambulances at a Bangkok hospital mortuary to make the journey to Uthai Thani, accompanied by their relatives.

Parents sobbed uncontrollably as they offered prayers for their sons and daughters at the site of the tragedy, where the convoy of ambulances stopped before making its way north.

Teachers and students at a number of schools around Thailand laid white flowers to mourn those killed in the blaze, believed to be the kingdom's deadliest road accident in a decade.

Thailand's Department of Land Transport said it was implementing urgent inspections of all natural gas-fuelled buses.

The department also will upgrade its safety guidelines to require crisis management training for drivers and safety inspection when such vehicles are commissioned by schools.

The department had suspended licenses for both the bus company and the driver.

AP/AFP/ABC