Mosques in the city have tunnels used by the militants to escape bombing and to store high-powered weapons

Galvez said the discovery of the tunnels is an indication that the gunmen are getting desperate and their fight is coming to an end. “Their tipping point is nearing,” he said.

With bombproof tunnels, anti-tank weapons hidden in mosques, human shields and familiarity with the terrain, the Maute gunmen are proving a far tougher opponent than the military had expected.

The roughly 10 percent of Marawi held by the militants has many tunnels and basements that can withstand 500-pound (227-kilo) bombs, military spokesman Lt. Colonel Jo-Ar Herrera said earlier.

He said even mosques in the city have tunnels used by the militants to escape bombing runs as well as to store high-powered weapons.

The military air strikes have spared mosques and Islamic schools known as madrassas, a limitation exploited by the militants.

Residents had built reinforced bunkers and tunnels underneath their houses after a 1970 Muslim uprising led to large parts of the city being razed.