By Agence France-Presse
Most of the dead were government workers enjoying a staff party when the fire broke out Tuesday afternoon in the multi-storey building in Hanoi's commercial Cau Giay district and quickly tore through neighbouring buildings.
Inspectors picked through rubble in the charred building Wednesday, where broken beer bottles were scattered and several motorbikes burnt to a crisp.
"Hundreds of firemen and police officers were mobilised to extinguish the fire," The Chung, a ward official told AFP, adding that two police officers were being treated for injuries sustained trying to put out the blaze.
State-run media quoted an official as saying 11 of the victims were government employees.
"They went out partying after finishing a test during a political training course," Hanoi's party chief Hoang Trung Hai said, according to Tuoi Tre newspaper.
He said the fire might have been sparked by construction inside the building and trapped patrons likely suffocated inside the windowless karaoke rooms.
Photos showed plumes of smoke billowing out of the building as firefighters in cranes doused the flames.
"I was carrying my stuff when I saw the fire suddenly start," said Do Thi Kha, 77, a food vendor in the area.
"The fire spread over to two or three neighbouring buildings before I saw fire trucks come in. It was terrible. I was terrified. They did not have enough water to extinguish the fire," she told AFP.
Another witness said the fire burned for more than five hours and was made worse when it ignited fuel tanks in cars parked nearby.
"Trucks had to keep coming back and forth with water as there was not enough. That was why it took so long to extinguish," said 69-year-old Nguyen Dinh Sau.
Prime Minister Nguyen Xuan Phuc has ordered police immediately to investigate the cause of the fire and "seriously punish wrongdoing", according to the government's website.
He also told local authorities to closely inspect karaoke bars and restaurants for fire safety and equipment.
Karaoke bars are popular hangouts in communist Vietnam, among government officials and young people alike, and Tuesday's fire was not the first of its kind in the capital.
In May 2014 five people burned to death at a karaoke bar in Hanoi after an electrical fault triggered a massive blaze.
Another karaoke venue in the city went up in flames in September this year, but no casualties were reported.
Blazes are common at houses, bars and restaurants in Vietnam, where fire prevention and firefighting is limited.