"We have recovered 27 bodies, others are being sent to hospital," said Prime Minister Anutin Charnvirakul, who visited the site of the fire that broke out around midnight on Sunday (3pm AEST).
Based on survivor accounts, he said the pub in Bangkok's Chatuchak district rapidly filled with smoke after a fire broke out, forcing many to run to the back of the venue near the bathrooms, but there were no fire escapes.
Another 63 people injured in the incident have been hospitalised, said Suriyachai Rawiwan, director of the Bangkok Metropolitan Administration's Department of Disaster Prevention and Mitigation.
"The number of deaths has stabilised at 27," he told Reuters.
Of those hospitalised, 22 are in critical condition, Bangkok Governor Chadchart Sittipunt told reporters.
By the time the rescue workers arrived at the scene of the accident, the fire that witnesses said ignited at the front of the venue's stage had already burned through the area, according to Suriyachai.
"We have to wait for the police to investigate," he added.
Firefighter Chakrit Khongkom, 45, said he arrived on the first fire truck to see the Rong Beer Na Lat Phrao pub alight and many patrons stranded inside, several of them trying to escape from the back of the venue. The few people coming through the front of the venue were burnt.
"The fire was not that aggressive, but the smoke had engulfed 100 per cent of the venue. The smoke was everywhere," he said.
"Most of the survivors were choking on smoke."
Body-camera footage from an emergency worker seen by Reuters showed firefighters in oxygen masks moving through the darkened remains of the pub with torches as they searched for survivors.
Several victims can be seen lying prone on the floor near the pub's toilets while rescue workers carry in stretchers.
The video also shows what appears to be the fire-damaged main room of the venue, with bar tables and stools still visible.
Chadchart said the pub had procured proper permits and had fire exits, but the fire spread rapidly and smoke filled the room, potentially making it hard for patrons to escape.
In the early hours of the morning, emergency personnel remained at the scene as bodies were prepared for transport and forensic teams collected evidence to help identify the victims.
Numbered bodies lay in two rows awaiting removal, surrounded by a large number of rescue workers. Firefighters stood near the pub's entrance, its facade scorched and blackened by the blaze.
A band member at the venue told local media that smoke suddenly filled the room after the lights briefly went out, followed by a massive explosion and fire.
"After the explosion I didn't see anybody trying to run, most of them were on the floor asking for help," he told reporters, his head still bandaged, without giving his name.
"I ran towards the door from the stage, about five metres. It was dark and there was smoke, no oxygen."
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