Explosions and gunshots were heard at the famous Resort World Manila in the Philippines on Thursday evening.
Armed police were deployed around the casino, prompting the officials to lockdown the resort.
"A masked gunman was on the second floor of one hotel, firing at guests," CNN quoted hotel employees as saying.
It was not immediately clear how many people may have been injured or killed.
Social media was abuzz with pictures showing plumes of smoke emanating from the Resort World Manila, which is close to the city's airport.
In a tweet, Resorts World Manila officials said the complex was in lockdown following reports of gunfire.
"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," Resorts officials also posted on Facebook.
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No one has claimed responsibility for Friday's attack.
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