The military's social media-monitoring team identified 63 accounts under fake names.
The Philippines army said on Friday it had called on Facebook to shut down dozens of accounts that Islamist militants have been using to spread misinformation about their attack on a southern town and to coordinate their battle with security forces.
The military's social media-monitoring team identified 63 accounts under fake names that it believed were being used by the Islamic State-allied Maute group and its sympathisers.
"They are spreading lies, they are spreading misinformation and they are creating more problems in our fight against the terrorists," spokesman Lieutenant Colonel Jo-Ar Herrera told a news conference in Marawi City, where at least 200 militants are still holed up 18 days after their attempt to capture the town.
Officials said an estimated 500-1,000 civilians were trapped in the built-up area occupied by the fighters, which came under heavy aerial bombardment on Friday.
Some civilians were being held as human shields, while others hiding in their homes fear capture by the militants and have no running water, electricity or food, said Zia Alonto Adiong, a politician involved in evacuation efforts.
Fake accounts are not permitted under Facebook, and in recent months the company has launched several initiatives against fake news.
"Our Community Standards do not allow groups or people that engage in terrorist activity, or posts that express support for terrorism," a Facebook representative said in an email response to a request for comment.
"Fake accounts are also prohibited. We will remove accounts and content that violate these policies when we are made aware of them."
One of the main Islamist factions dug in around the heart of the city is the Maute group, a relative newcomer on the insurgency-plagued island of Mindanao that analysts say is more sophisticated and media-savvy than more established groups.
"Part of what we are looking at is radicalisation in social media. This is used to radicalise the youth," Herrera said. "We see massive misinformation and use of social media to facilitate propaganda activities."
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The seizure of Marawi City has alarmed Southeast Asian nations which fear Islamic State, facing setbacks in Syria and Iraq, is establishing a stronghold in Mindanao that could threaten the whole region.
About 40 foreigners have fought alongside the Philippine militants in Marawi City, most of them from Indonesia and Malaysia though some came from the Middle East.
The Philippines is largely Christian, but Mindanao has a significant population of Muslims and Marawi City is overwhelmingly Muslim.
Maute joined forces with Isnilon Hapilon, who was last year proclaimed by Islamic State as its Southeast Asia "emir". Military officials said they believed Hapilon and the two founding brothers of the Maute group were still in the town.
Officials said the army was chipping away at the territory held by the militants. The volume of fire coming from the rebel side had dropped and their sniper firing had become more selective.
However, the army had to proceed carefully because civilians were being held in mosques and madrasas, or religious schools, and the fighters had prepared for a long siege, stockpiling arms and food in tunnels and basements.
According to the latest official figures, 138 militants had been killed, with the death tolls for security personnel and civilians at 40 and 20, respectively.
Military spokesman Restituto Padilla said the aim was to end the siege by Monday, the Philippines' independence day.
"The chief of staff made an announcement, hoping, that by Monday we can freely wave our flags in every corner of Marawi and we are working feverishly to do that," he said.