Preliminary investigations by Bangkok's disaster mitigation department suggest the fire could have been caused by an electrical short circuit in an air conditioner, though no official cause has been given.
Authorities have promised a thorough investigation.
This is not the first time such an incident has occured in Thailand.
Despite official promises to improve fire and electrical safety standards following previous incidents, they are still often poorly enforced.
Prime Minister Anutin Charnvirakul, who visited the scene early on Monday, said a musician who was performing when the fire broke out told him "there was blasting and everybody tried to flee from the smoke and flames".
Many people did not make it out of the bar because they "went to the back of the building and tried to hide themselves... in the toilet", the musician said, according to Anutin.
Firefighters, who were alerted to the blaze by a passing driver, were reportedly able to bring the flames under control in about half an hour.
However, despite their efforts, nine men and 18 women were killed. More than 60 are now being treated in hospital, including 22 who are in critical condition, authorities said.
Initial findings suggest the majority of the victims had died from smoke inhalation, said Suriyachai Raviwan, the director of Bangkok's disaster mitigation department.
Bangkok governor Chatchart Sittipunt say the flammable interior decorations on the bar's ceiling may have facilitated the blaze's rapid spread.
There are also reports of people found unconscious near the building's emergency exit, suggesting that there may have been some obstruction, the governor added.
Authorities say these suggestions can only be confirmed through further investigation by forensic officers.
One motorcyclist, Surin Jaiharn, told AFP that he helped about five people flee the burning bar using clothing to extinguish flames on their bodies.
"I feel depressed. I saw many deaths and I do not know the fate of the people I helped," he told AFP.
The driver who alerted the fire department told Thai news outlet the Daily News that he broke windows to help two people escape.
As of Monday morning, the bar has been cordoned off, with shattered windows and furniture piled up outside its entrance. Confronting pictures have emerged of many body bags lined up outside the bar.
Inside, the furniture, walls and ceiling are completely blackened, with parts of the ceiling peeled off. When the BBC arrived at the scene on Monday, an acrid smell of burning lingered in the air.
