The death toll from a devastating bar fire in Bangkok has risen to 33, authorities said on Wednesday, as Thailand pledged a nationwide overhaul of safety regulations for entertainment venues.

The blaze tore through the popular Rong Beer Na Lad Phrao restaurant and bar on Sunday night, rapidly engulfing the venue.

Investigators are examining the cause of the fire and why it proved so deadly, with many of the victims found near the toilets.

In a video released by Prime Minister Anutin Charnvirakul's office showing his visit to the scene, seen by AFP, an official told him there were two functioning exit doors.

However, one of the doors was marked "staff only", with no clear indication that it could be used as an emergency exit, the official said.

"So this means people weren't aware that this was the emergency fire exit," Anutin was heard saying.

One of the rescue workers replied: "This might have been to prevent customers from leaving without paying."

Speaking on Tuesday, Anutin suggested that another door that could have been used as an escape route had been locked, saying: "How could this happen?"

The government has ordered an urgent inspection of entertainment venues across the country.

Anutin also said authorities would review regulations governing how such venues are licensed after it emerged that Rong Beer Na Lad Phrao was not licensed to host live music, despite regularly doing so.

"Businesses that do not comply with regulations are not sustainable, and the owner of this venue will face the fullest extent of the law," the prime minister said.

The owner of the bar was admitted to intensive care immediately after the fire, although it was not immediately clear whether he remained in hospital.

Thailand's enforcement of health and safety regulations, particularly in bars and nightclubs, has long been a source of concern.

In 2009, a fire ripped through Bangkok's Santika Club during New Year celebrations, killing 67 people and injuring more than 200.

A building safety expert told AFP earlier that Rong Beer Na Lad Phrao lacked the safety systems required to accommodate the large crowds and live music events it regularly hosted.

The Erawan Emergency Medical Centre, operated by Bangkok city authorities, said on Wednesday that the death toll had risen to 33. Around 70 people were injured, including more than a dozen who remained in critical condition in hospital.