MANILA - The Bureau of Fire Protection on Monday said it is still investigating if welding work caused a fire that left 5 people dead at Waterfront Manila Pavilion, a hotel-casino complex in the capital's Ermita district.

A slot machine operator had claimed that the inferno, which raged for 25 hours starting Sunday, started amid welding work near an escalator.

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"Iyan pong impormasyon na iyan ay hindi kumpirmado... Kailangan po talagang i-verify po natin, masusi at siyentipiko ang ating pangangalap," fire marshal Superintendent Jonas Silvano told radio DZMM.

(That information is not yet confirmed. We have to verify it, our investigation should be thorough and investigation.)

Initial investigation indicated that the blaze started at the casino area in the first floor of the 22-story Manila Pavilion, he said.

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The dead were all casino employees, and another of the workers was still in critical condition. About 20 people were injured.

About 300 guests and staff were evacuated safely, hotel and fire officials told a press conference.

Some firefighters were treated after inhaling the dense smoke, which shrouded the chaotic scene on Sunday.

"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," Silvano had said in another radio interview.

The BFP said it would check reports from survivors that the sprinklers and alarm system of the 50-year-old hotel did not work.

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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 4-story shopping mall in the southern city of Davao.

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With a report from Agence France-Presse