Witnesses have said they are dressed in black and wearing masks.
Philippine police, firetrucks and SWAT teams rushed to Resorts World, where witnesses reported seeing a masked gunman firing at guests on the second floor of a hotel.
But dela Rosa said: "We can not attribute this to terrorism".
The Philippines Red Cross tweeted that it had transported three people from the resort to hospitals. "People were screaming, guests and employees were in panic", Navaro said.
Trump, before announcing the United States was leaving the Paris climate agreement, opened the event by saying "our thoughts and our prayers" are with those affected by the "terrorist attack in Manila".
Most of the dead died of suffocation, the ANC news channel reported, quoting sources at the Bureau of Fire Protection.
Angelique Kerber upset by Ekaterina Makarova at French Open
Kerber has not been playing well, and she came into the French Open with little confidence that she'd fare well on clay. The top seed dropped serve six times and made 25 unforced errors in a match that lasted an hour and 22 minutes.
Officials were quick to speculate that the attack was the work of Islamic State as authorities have been fighting ISIS in parts of the country.
"But one woman I was trying to save fell from the second floor".
Early reports suggested the gunman had been shot, but police later said he'd committed suicide by setting himself alight. "We will investigate later". We have zero visibility at makapal pa rin usok sa (there is still thick smoke emanating) floors so we have to be very careful with every move.
Police said there had been no confirmed fatalities but 30 people had been injured in the stampede and from inhaling smoke.
It was not immediately clear if anyone was killed, or if the attack had concluded.
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. "We are closely monitoring the situation", Trump said in the White House Rose Garden.
Manila's chief of police, Oscar Albayalde, said: "It's either he lost in the casino and wanted to recoup his losses or he went totally nuts".
Tensions have run high in recent weeks, as Filipino president Rodrigo Duterte has faced an escalating conflict with Islamic State-affiliated militants, known as Abu Sayyaf, in the southern Filipino city of Marawi, on the island of Mindanao.