MANILA (AFP) - The Philippine military on Friday (June 9) asked Facebook to close dozens of accounts linked to Islamist militants who attacked a southern city, saying they were being used for propaganda.

Military spokesman Lieutenant Colonel Jo-ar Herrera said the armed forces' social media unit uncovered 63 Facebook accounts belonging to the hardliners and their supporters.

"These 63 accounts are spreading malicious information and misinformation that affect the information landscape and the mindset of every Filipino," Lt Col Herrera told a news conference in the southern city of Marawi.

The military has asked Facebook Philippines "to conduct necessary measures to take down these fake accounts", Lt Col Herrera said.

Hundreds of militants waving the black flag of the Islamic State in Iraq and Syria (ISIS) rampaged through Marawi on May 23 and seized parts of the city.

They have so far fended off more than two weeks of air and ground assault by government forces.

President Rodrigo Duterte said the attack was part of a wider plot by ISIS to establish a base in the southern region of Mindanao, and declared martial law there to quell the threat.

Forty government troops and 138 militants, have been confirmed killed due to the fighting, according to Lt Col Herrera.

Twenty civilians have also been confirmed killed but the true number is likely to be higher with authorities yet to fully assess the roughly 10 per cent of Marawi still being held by the militants.

About 2,000 people are believed to be trapped in the militant-held areas with some being used as human shields, the military has said.

ISIS militants have made extensive use of social media to strike fear and spread their ideology, and analysts have said many of their followers were radicalised by reading and listening to online propaganda material.

When asked about the military's request, Facebook told AFP it would remove accounts that promoted terrorism.

"We want to provide a service where people feel safe. That's why we have community standards that explain what you can and cannot do on our service," a Facebook statement said.

"Our community standards do not allow groups or people that engage in terrorist activity, or posts that express support for terrorism. Fake accounts are also prohibited."