National police chief Ronald dela Rosa said: "Don't panic, this is not a cause for alarm".
PNP-National Capital Region spokesman Chief Inspector Kimberly Molitas said in a statement that the gunman has been identified and relayed news that some people may have died due to suffocation.
Gunshots and explosions erupted after midnight in a hotel and casino complex near Manila's airport, sending hundreds fleeing into darkened streets early Friday in what police said was an attack by a lone gunman with no apparent links to terrorism.
Police report serious injuries, along with a fire started by the gunmen.
Police confirmed there were reports of gunfire at Resorts World, which is across a road from one of the main terminals of the Philippines' worldwide airport. "They briefed him that the media was reporting that ISIS was claiming responsibility".
The president called the incident a "terrorist attack" before addressing the media about the US withdrawal from the Paris Climate Treaty.
"We are searching the area as much as we can, to make sure it is clear", a Resorts World official, Stephen James Riley, told reporters.
Resorts World Manila said the shooting triggered a lockdown, and that it's working with police in the case. Authorities earlier said they think robbery was the motive.
"I was about to return to the second floor from my break when I saw people running".
The General said one gunman is believed to be behind the attack, in which a security guard was shot and 75 people were reported injured, today.
The incident was quickly claimed by ISIL, which said "lone wolf soldiers" from its group carried out the attack, according to the Site Intelligence Group that monitors terrorist organisations.
"It's too early to tell but as far as we are concerned there was no indication the IS-affiliated militants were behind the shooting", he said.
"I heard many, many gunshots", Julio Silva, a witness who managed to dash out of the mall complex, told the DZMM radio network.
He says the gunman apparently killed himself.
Witnesses say gunshots and explosions have been heard at a mall. "It's really very sad what's going on throughout the world with terror", he added.
A total of 120 militants have been killed in the fighting since May 23, when a failed government raid to capture one of Asia's most-wanted militants, Isnilon Hapilon, triggered the siege of the city by the rebels.
"We remain in close contact with local officials", she said, adding that USA citizens were advised to exercise caution and monitor local media for further information.
The battle for Marawi, a largely Muslim city, has displaced at least 70,000 residents and left 140 people dead.