Philippines police chief General Ronald Dela Rosa said there was no evidence the attack was terrorism and only one gunman was behind the attack.
The gunman stole gambling chips, shot an LED monitor and set gambling tables ablaze by pouring gasoline on them, Dela Rosa said.
But he said the assailant did not fire at people.
It was not clear how the gunman smuggled enough petrol into the crowded casino to cause explosions and what prompted Dela Rosa to rule out terrorism so quickly.
Witnesses reported seeing injured people, including a Swat officer who rushed to the scene and was reportedly fired upon. AP is trying to confirm those reports.
About 90 minutes after the attack began, Resorts World Manila said in its Facebook account that it was "on lockdown following reports of gunfire from unidentified men", adding: "The company is working closely with the Philippine National Police to ensure that all guests and employees are safe.
"We ask for your prayers during these difficult times," the company said.
EXPLOSION JUST HAPPENED RESORTS WORLD MANILA!!
I'm ok everyone!!!!
Posted by Tikos Low on Thursday, 1 June 2017
Philippine police cordoned off the area near the Ninoy Aquino International Airport.
"I heard many, many gunshots," Julio Silva, a casino player who managed to dash out of the mall complex amid the gunfire, told DZMM radio network.
Silva said he saw a Swat member shouting: "I was hit, I was hit."
A DZMM radio reporter said she saw two ambulances ferrying two people away, one with a bandage on the leg and another in wheelchair.
As news of the attack spread, US President Donald Trump offered the thoughts and prayers of the American people to the Philippines.
"It is really very sad as to what's going on throughout the world with terror," he said from the White House Rose Garden.
Trump said he was "closely monitoring the situation" and would continue to provide updates.
The US' Site Intelligence Group, which monitors terrorism, said an Isis-linked Filipino operative who provides daily updates on the ongoing clashes in Marawi claimed "lone wolf soldiers" of the Isis were responsible for the attack.
An English message by the operative was distributed across several pro-Isis Telegram chat groups, Site said.
According to Site, he wrote: "The lone wolf soldiers of Khilafah attack the heart of Kufar the city of Manila in Resort World."
Earlier, President Rodrigo Duterte sent soldiers to the southern Philippines to end a siege by about 500 militants.
Officials say 120 militants and at least 25 soldiers, five policemen and two dozen civilians have been killed in the fighting since early last week.
Duterte declared martial law in the Mindanao region, the southern third of the Philippines, to crush the insurrection and poured in troops backed by airstrikes, artillery fire and armoured vehicles.The unrest has boosted fears that Isis' violent ideology is gaining a foothold in the country's restive southern islands, where Muslim separatist rebellions have raged for nearly half a century.