A huge fire in a Bangkok bar killed at least 30 people and injured dozens, officials and witnesses said on Monday, the deadliest such incident in Thailand in almost two decades.

People ran screaming out of the venue on the outskirts of the Thai capital on Sunday evening as flames shot out of the entrance, several with their clothing burned or ablaze, according to video verified by the AFP news agency.

An AFP journalist saw several body bags on the ground early on Monday morning outside the Rong Beer Na Lat Phrao bar and restaurant. A few hours later, forensic police began combing their way through the devastated venue.

"The fire spread very quickly, reaching up to the ceiling. Smoke was likely the main cause of death," Bangkok Governor Chadchart Sittipunt told reporters at the scene.

He said 27 people were killed and 63 were hospitalised with injuries, including 22 in critical condition, adding that authorities had launched an investigation.