Rescuers said billowing smoke, sometimes so dense it obscured the 22-floor Waterfront Manila Pavilion, had hampered their efforts to contain the blaze which started Sunday morning.
The dead were all casino employees, and another of the workers was still in critical condition. About 20 people were injured.
Some 300 guests and staff were evacuated safely, hotel and fire officials told a press conference.
"We are hoping no one was left behind in the rooms. Our firefighters have not yet gone up all parts of the building," Metro Manila Development Authority acting chief Jojo Garcia said.
"The smoke was so big, so you can just imagine, there was zero visibility and our firefighters had difficulty breathing. Even outside the building there was zero visibility and it was much harder to operate inside," Manila district fire marshal Jonas Silvano told radio DZBB.
Deadly blazes break out regularly in the Philippines, particularly in slum areas where there are virtually no safety standards.
The deadliest in recent years was in suburban Manila where 72 people died in 2015 at a factory which makes rubber sandals.
But more modern buildings have also gone up in huge, deadly blazes.
In December, 38 people died in a fire at a four-storey shopping mall in the southern city of Davao.
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