Hundreds of people were in the Grand Diamond City building in the border town of Poipet when the fire broke out after 23:30 (16:30 GMT) on Wednesday.
Video circulating on social media showed people apparently jumping or falling from upper floors.
The hotel-casino employs 400 workers and had many Thai nationals as guests.
Many of the victims are thought to have been trapped on upper floors and a Thai volunteer organisation, Ruamkatanyu Foundation, said it was unlikely that they could have survived.
“There are 19 dead so far as we see bodies and bones,” said Sek Sokhom, director of Cambodia’s Banteay Meanchey provincial information department. He warned that the death toll could be higher.
Rescue teams had struggled to reach people inside the building because of the dense smoke and are continuing to search it for those who are missing.
Poipet is a key crossing point between the two countries. It is also popular for its casinos, which many Thai nationals visit because gambling is mostly illegal in Thailand.
The building backs on to the Thai border and many of the injured were sent across the border for treatment. Some were sent to a hospital in the nearby town of Aranyaprathet.
However, the exact number of people injured is unclear – with the numbers given by local officials and quoted by the media varying greatly. It’s thought to be in the dozens.
Authorities are still working to determine the cause of the blaze.
Source : BBC