MARAWI CITY - Islamist militants locked in street-to-street battles with security forces in this southern city have killed 19 civilians, the military said Sunday, bringing the official death toll from nearly a week of fighting to at least 85.

The violence prompted President Rodrigo Duterte to declare martial law on Tuesday across the southern third of the Philippines to quell what he said was a fast-growing threat of the Maute group.

Authorities said the militants had killed 19 civilians in Marawi, a mostly Muslim-populated city of 200,000 people. These included three women and a child who were found dead near a university.

"These are civilians, women. These terrorists are anti-people. We found their bodies while conducting rescue operations (on Saturday)," regional military spokesman Lieutenant Colonel Jo-ar Herrera told AFP.

An AFP photographer saw another eight bodies by a road in the outskirts of Marawi on Sunday, with local residents identifying them as employees of a rice mill and a medical college.

Herrera said the military had yet to investigate the reported deaths.

The violence began when dozens of gunmen went on a rampage throughout Marawi after security forces attempted to arrest Abu Sayyaf leader Isnilon Hapilon.

Thirteen soldiers, two policemen and 51 militants have died in the fighting, according to authorities. This brings the combined official death toll to at least 85.

Most of the city's residents have fled because of the fighting, which has seen the military heavily bomb residential areas where the militants were believed to be hiding.

The military announced on Saturday, the start of the Holy month of Ramadan, that it would intensify the bombing campaign.

"In as much as we would like to avoid collateral damage, these rebels are forcing the hand of government by hiding and holding out inside private homes, government buildings and other facilities," military spokesman Brigadier General Restituto Padilla said.

"Their refusal to surrender is holding the city captive. Hence, it is now increasingly becoming necessary to use more surgical airstrikes to clear the city and to bring this rebellion to a quicker end."

© Agence France-Presse