The SITE Intelligence Group, a USA terrorism monitor, said an Islamic State group-linked Filipino operative who provides daily updates on the clashes in Marawi claimed IS "lone wolf soldiers" were responsible for the attack.
Earlier, authorities said 35 dead victims were found in the casino area in Resorts World Manila after a lone gunman fired shots and set fire to gambling tables in the early hours of Friday morning.
Thirty-six bodies were found inside a casino in the Philippine capital today after a masked man burst in with an automatic rifle and set fire to a gaming room, authorities said, but insisted it was not a terrorist attack.
Presidential spokesman Ernesto Abella has ruled out a link between the casino attack and the ongoing operation against Islamist militants occupying several neighborhoods of Marawi, who are seeking recognition from Islamic State.
The police chief said 18 of the 54 injured people were in the hospital, while others sustained only minor injuries.
Oscar Albayalde, Manila's chief of police, said: "It's either he lost in the casino and wanted to recoup his losses or he went totally nuts".
Taiwan's Foreign Ministry said four Taiwanese were among those killed, and a South Korean Foreign Ministry official said one South Korean had died, apparently after a heart attack.
The man, who has yet to be named, entered the casino shortly after midnight and began firing an assault rifle, sparking widespread panic. Police later searched a auto and found registration information they haven't made public.
As the gunman left, he exchanged shots with a building guard who managed to shoot him in the leg after being wounded, police and casino officials said.More news: Manchester United defeats Ajax to win Europa League
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Video of the scenes showed guests frantically running for the exits, the sound of gunshots and smoke coming from the upper floors of buildings.
"For now, we can say that this is not an act of terror". He did not hurt the guard but went straight to the gambling area, dela Rosa said.
The management said 22 guests and 13 of its employees died. "Some of them jumped from the second floor just to escape".
The complex, which is described on RWM's website as "the first and largest integrated resort in the Philippines", is located across from Ninoy Aquino International Airport.
MOFA said the victims had been identified, while their relatives would be assisted in flying to the Philippines.
Police spokesperson Kimberly Molitas said authorities recovered 113 million Philippine pesos ($2.27 million) worth of stolen casino chips.
"He would have shot all the people gambling there" if it had been terrorism, said the national police chief, Gen. Ronald dela Rosa.
But police quickly said they did not believe the attacker had any militant connections.
Police said it was not a terrorist attack because the gunman did not threaten or shoot anyone.
President Rodrigo Duterte declared martial law on the southern island of Mindanao last week and warned it could become a haven for supporters fleeing Iraq and Syria.