The Philippine military said on Wednesday, June 28 it was likely that large numbers of civilians had been killed during the five-week occupation of a southern town by Islamist rebels, due to "atrocities" by the militants allied with Islamic State, Reuters reports.

A toll of 27 Marawi residents killed was only what the authorities could independently confirm and a "significant number" of dead had been seen by those who had escaped fighting between insurgents and government forces, said military spokesman Restituto Padilla.

"The number you have right now is 27, (it) may increase significantly once we are able to validate all this information," he told a news conference.

"There have been a significant number that have been seen but again, we cannot include many of these."

He said the cause of those deaths would be "atrocities committed by the terrorists".

Among those atrocities, the army says, have been the forcing of residents to loot homes, take up arms, or become sex slaves.

The military has been reluctant to discuss the possibility that the real impact of the fighting on civilians could be far more severe than has been reported.

It has played down the impact of daily air strikes and mortar assaults aimed at rebel sniper positions, which have reduced areas of the lakeside town to rubble and alarmed people stuck there, some of whom have said the shelling was a bigger threat than the militants.