"The number of deaths may reach more than 20," said Sek Sokhom, head of the Banteay Meanchey provincial information department.
Sixty people had been injured, he said.
A Thai rescuer at the scene said they recovered 14 bodies before the figure was later updated to 19.
Rescuers struggled to extricate people who feared being trapped.
Khieu Sopheak, an interior ministry spokesperson, had earlier said it was not clear how many people were still inside the charred building.
About 400 employees and patrons were in the Grand Diamond City casino and hotel in the town of Poipet when the fire broke out around midnight, and up to 30 were still missing.
A key part of Cambodia's tourism industry, important for its economy, casinos in the capital of Phnom Penh and on the borders with Vietnam and Thailand are a draw for visitors from Asian nations that ban gambling.
Casinos in Poipet are hugely popular with short-term Thai visitors in particular as gambling is illegal and unlicensed casinos operate underground in the neighbouring country.
At least 25 people were being treated in hospitals in Sa Kaeo province across the border, Thai authorities said.
Reuters