The gunman stole gambling chips, shot TV screens and set gambling tables ablaze by pouring gasoline on them, dela Rosa said.
Serious injuries have been reported but it's unclear how many people may have been injured or killed. Dones said employees told him they saw two suspects wearing black and carrying long firearms, however police later said there was just one suspect.
But he said the assailant did not fire at people he encountered.
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Resorts World Manila, in the southwestern part of metropolitan Manila near Ninoy Aquino International Airport, said on Twitter early Friday that it was working closely with police to ensure the safety of employees and guests, adding, "We ask for your prayers during these hard times".
The unrest has boosted fears that the Islamic State group's violent ideology is gaining a foothold in the country's restive southern islands, where Muslim separatist rebellions have raged for almost half a century.
Smoke rises from the Resorts World Manila complex early Friday, June 2, 2017 in Manila, Philippines. The police is on top of the situation.
Police chief Ronald Dela Rosa ruled out terrorism as the motive of the attack as the suspect neither threatened nor harmed civilians.
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Resorts World Manila is the largest casino resort in the country, according to its website.
Reports from eyewitnesses were more dramatic.
SITE INTEL GROUP, which reports on jihadist activity, tweeted that a Filipino operative for the Islamic ISIS, has said the group is responsible for the attacks.
"I live right behind Resorts World. and you can see the smoke and hear gunshots and people are shouting at each other", one witness said in a video posted on Facebook.
"We ask for your prayers during these hard times", the company said. He said he is "closely monitoring the situation" and will continue to provide updates.
Shots and loud bangs rang out shortly after midnight in the buildings close to Terminal Three of Ninoy Aquino International Airport and an air force base.
Philippine President Rodrigo Duterte imposed martial law last week across the southern region of Mindanao to crush what he said was a rising threat of Islamic State there.