Australia is helping the Philippines get a bird's eye view of Islamist militants occupying a city in the country's south.

The federal government has sent two spy planes to surveil sea and land approaches to Marawi amid the threat of terrorism.

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While the P-3 Orions are capable of picking up on-ground communications, the Royal Australian Air Force is - for now - focused on taking photos and videos.

"Mainly it's a collection of imagery to be able to provide an overhead view which we then provide to the Filipino forces," joint operations chief David Johnston told reporters in Canberra on Thursday.

More than 500 militants, including foreign fighters, stormed the besieged city in May.

Vice Admiral Johnston said it was unlikely other parts of the island of Mindanao or the country will experience similar events, given the number of fighters and weapons involved.

He also raised doubts about the credibility of reports that foreign fighters were moving into the Philippines to join and expand the fight.

"I don't think there's been a significant movement of fighters from outside of the region. There have been some, but generally, in relatively small numbers."