A bus carrying dozens of primary school children has crashed and caught fire just outside the Thai capital of Bangkok. Sixteen children and three teachers are reported to have escaped, but 22 pupils and three teachers are still unaccounted for, according to the country"s transport minister. Thailand"s prime minister said the accident resulted in "deaths and injuries" - but the exact number of fatalities has not yet been confirmed. Photographs show the bus completely destroyed by the fire. Investigators are said to have been unable to enter the vehicle because of the heat, according to local media. Eight of the 19 people who managed to escape were sent to hospital for treatment, a health ministry official said. The bus was one of three that were carrying children and teachers returning from a school field trip in the northern province of Uthai Thani. Transport Minister Suriyahe Juangroongruangkit said the bus was powered by "extremely risky" compressed natural gas. "This is a very tragic incident," Suriyahe told reporters at the scene. "The ministry must find a measure... if possible, for passenger vehicles like this to be banned from using this type of fuel because it’s extremely risky." Thailand"s prime minister, meanwhile, has ordered ministers to visit the scene. "As a mother, I would like to express my deepest regrets to the families of those killed," Paetongtarn Shinawatra said. "The government will be responsible for all the medical costs and the compensation for those killed," she added. Piyalak Thinkaew, who is leading the search, said the bodies of those killed were so badly burned, it was hard to identify them. "Some of the bodies we found were very, very small," he told reporters at the scene, adding that the fire started at the front of the bus. "The kids" instinct was to escape to the back so the bodies were there," he said. The bus was travelling on a highway into Bangkok when a tyre burst, sending it crashing into a barrier, a rescue worker said in footage broadcast on local television. Video footage from the scene showed flames engulfing the bus as it burned under an overpass, huge clouds of dense black smoke billowing into the sky. The driver has fled the site of the crash but authorities are confident he will be tracked down, according to Thailand"s Interior Minister Anutin Charnvirakul, who spoke to reporters at the scene. The ages of the children on board remain unclear, but the school has pupils between three and 15 years old. Thailand has one of the worst road safety records in the world, with unsafe vehicles and poor driving contributing to roughly 20,000 fatalities a year. — BBC