People ran screaming out of Resorts World Manila, which is across a road from one of the main terminals of the Philippines' global airport, after the man fired what police chief Ronald dela Rosa said was an M4 assault rifle.
Local media reported gunmen inside the building, gunshots and explosions. Dela Rosa said the gunman had not aimed his weapon at resort-goers and appeared more concerned with getting a hold of "casino chips".
Witnesses reported seeing injured people, including a SWAT member, who rushed to the scene and was reportedly fired upon by unidentified men.
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Multiple gunshots were heard as the men reportedly opened fire on guests, and witnesses claim the attackers set fire to casino tables.
The incident in Manila comes as the entire country is on high alert following the attack by the Maute Group in the southern city of Marawi in Mindanao, which left over 100 people dead, including many civilians.
But the resort complex then said on Twitter that police operations were still "ongoing" and that they did not have "official information as to the identity or affiliation of the persons responsible for the attack". According to SITE, he wrote: "The lone wolf soldiers of Khilafah attack the heart of Kufar the city of Manila in Resort World".More news: Penguins beat Predators, take 2-0 Stanley Cup lead
The security guard shot was the waist by the gunman and about 75 others suffered mostly minor injuries such as bruising as they stampeded to get out, said police officer Jeffrey Francisco.
Gunshots and explosions rang out in the early hours of Friday morning at a luxury resort complex in the Philippines, as smoke rose from a portion of the building.
The complex, opposite the Ninoy Aquino International Airport, includes four hotels, casinos, a shopping centre, cinemas, restaurants, clubs and a theatre.
"We are still investigating the situation", Reilly said in a statement.
Philippines President Rodrigo Duterte declared martial law over the island of Mindanao in light of the crisis.
"I was about to return to the second floor from my break when I saw people running". He said he is "closely monitoring the situation" and will continue to provide updates.
The incident is unfolding amid a 10-day deadly siege by Muslim militants aligned to the Islamic State group in the southern city of Marawi.