Warning of a major attack on the Islamic State group's capital, aircraft flying over the Syrian city of Raqqa on Friday dropped were dropping thousands of leaflets urging residents to flee. get out

"It’s time to leave the city," reads one of the leaflets, which shows showing an sketch image of people running away from fleeing a black-and-white ISIS sign and into an open green field. The images were being shared appeared, according to photos posted on social media.

U.S. military Defense Department officials in Washington declined to comment on the leaflets Friday afternoon.

The Daily Beast reported that the leaflets were being distributed coming from U.S. warplanes, and quoted an American official who suggested they're being used as a form of may reflect psychological warfare — "It’s part of our mess-with-them campaign," the official said — rather than to warn of an imminent attack.

On Wednesday, a spokesman for U.S. troops in Iraq said the American-backed Syrian-Arab Coalition, or SAC, has been growing and will target Raqqa eventually.

"The SAC is continuing to gain strength, it's continuing to grow, and we'll continue to build up in posture for an eventual move against Raqqa. That's what our focus is," said Army Col. Steve Warren, the spokesman for Operation Inherent Resolve.

In April Defense Secretary Ash Carter announced plans to send more U.S. troops to Syria, expanding the force of special operations troops from 50 to 300. Those troops will advise friendly militias and help coordinate U.S. air strikes with the militant's ground attacks on ISIS.

U.S. warplanes dropped leaflets over Raqqa in July 2015 that declared "Freedom is coming."