JAKARTA (THE JAKARTA POST/ASIA NEWS NETWORK) - Jakarta Governor Djarot Saiful Hidayat has urged the public to be more vigilant after an Islamic State in Iraq and Syria (ISIS) flag was found at the police headquarters in South Jakarta.

An ISIS flag had been placed on a fence surrounding the Kebayoran Lama police headquarters on Tuesday (July 4).

An unknown individual riding on a motorcycle had reportedly stopped briefly in front of the headquarters to place the 100cm by 50cm flag on the fence.

The police also found a letter that threatened to turn Jakarta into a war zone like the Philippine city of Marawi, which has been the scene of battles between the military and ISIS-linked terrorist groups which had rampaged across the city in May.

"This is a warning to the police and citizens to be more vigilant," Djarot said in Jakarta on Tuesday.

The situation at the police headquarters has returned to normal, the Kompas.com website reported.

"The service should run normally and the incident should not hamper our activities," Kebayoran Lama Police chief Comr. Ardi Rahananto said.

The police are currently investigating the incident by checking CCTV camera footage and searching for witnesses.