Gunfire was reported at a hotel and a casino resort in the Philippine capital on Friday, the operator of the complex said, and the Islamic State group (IS) quickly claimed responsibility.
"Resorts World Manila is currently on lockdown following reports of gunfire from unidentified men," the company said on its Twitter account.
"The company is working closely with the Philippine National Police to ensure that all guests and employees are safe."
IS said "lonewolf soldiers" from its group carried out the attack, according to the SITE Intelligence Group that monitors terrorist organisations.
Police confirmed there were reports of gunfire at Resorts World, which is across a road from one of the main terminals of the Philippines' international airport.
There were no immediate reports of fatalities or injuries.
Philippine President Rodrigo Duterte imposed martial law last week across the southern region of Mindanao to crush what he said was a rising threat of Islamic State there.
He declared martial law shortly after militants went on a rampage through the southern city of Marawi, which is about 800 kilometres (500 miles) south of Manila.
Security forces are still battling the militants in Marawi, and the clashes there have left at least 171 people dead.
Duterte said last week he may need to declare martial law across the rest of the country if the terrorism threat spread.