With bomb-proof tunnels, anti-tank weapons hidden in mosques, human shields and a "mastery" of the terrain, Islamist militants holed up Marawi City are proving a far tougher opponent than military chiefs expected.
Two weeks after gunmen waving black flags of the Islamic State (IS) group rampaged through Marawi, initial assertions from authorities that the conflict would be over in days have given way to warnings of a protracted battle.
"The advantage of the (enemy) is their mastery of the terrain. They know where even the smallest alleys lead to and they are free to go around," Major Rowan Rimas, an operations officer for the Marines, told reporters in Marawi this week.
"They know where the government forces are coming from and where they are taking cover. They have snipers and their positions are well-defended."
Defense Secretary Delfin Lorenzana admitted at the start of the conflict that security forces were taken by surprise when dozens of gunmen appeared on the streets of Marawi following a failed raid to capture one of their leaders.
They emerged from homes in Marawi, the biggest Islamic city in the mainly Catholic Philippines with 200,000 residents, and went on a rampage that included taking a Catholic priest hostage, opening up two jails and destroying many buildings.
Lorenzana and military leaders have since said they unexpectedly interrupted plans by the militants to take over Marawi in a spectacular event to show that IS had arrived in the Philippines and was intent on carving out a local territory.
They initially estimated there were about 100 gunmen but later said there were as many as 500, supplemented by foreign fighters including from Chechnya, Saudi Arabia and Yemen.
The fighters have also had a surprisingly large arsenal of military hardware, including rocket-propelled grenades and a so-far inexhaustible amount of ammunition for high-powered assault rifles.
Authorities said two brothers from Marawi surnamed Maute were key leaders in the battle, giving the militants a crucial home-ground advantage that has allowed them to withstand a relentless air campaign of bombs and rocket strikes.
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 Lieutenant-Colonel Jo-ar Herrera told AFP on Monday.
"Even mosques here have tunnels," Herrera said, adding the fighters were using them to escape the bombing as well as to store high-powered weapons.
"These are all part of the dynamics of the battlefield that makes it more difficult for us."
Complex, extensive operation
Herrera said Philippine military protocol called for mosques and Islamic schools known as madrassas to be spared from airstrikes and artillery, a limitation exploited by the gunmen.
Residents built reinforced bunkers and tunnels underneath their houses after a 1970 Muslim uprising led to large parts of the city being burnt, Norodin Alonto Lucman, a senior Marawi politician, told AFP.
The Mautes are in partnership with Isnilon Hapilon, the target of the initial raid whom the military regards as the IS "amir," or its top leader in the Philippines. Military chiefs said they believed he was still in Marawi.
Adding to concerns that the militants were running a far more complex and extensive operation than authorities initially believed was the discovery on Monday in a house the gunmen had been using of 52.2 million pesos ($1.05 million) in cash.
The gunmen have the added insurance of about 2,000 trapped civilians serving as virtual human shields, along with the priest and more than a dozen civilians kidnapped at the start of the crisis.
President Rodrigo Duterte imposed martial law across the entire southern region of Mindanao, home to 20 million people, within hours of the crisis erupting in a bid to quickly crush what he said was the fast-rising threat from IS.
He set a target late last week of Tuesday to clear the city of the militants.
As that deadline passed, Duterte issued typically aggressive rhetoric to soldiers.
"I am ordering you to crush our enemy," he said in a speech to troops in the south,
"When I say crush them, you have to destroy everything, including lives."
But a quick military victory looks increasingly unlikely.
Lorenzana, who saw his own 10-day target pass on the weekend, said battles in recent years with Islamist militants in the south typically led to them running away but that this time they were showing far more resolve.
"Normally in this kind of conflict the local fighters will just scamper away and maybe hide in the mountains," he said while on a visit to Marawi on the weekend.
"But surprisingly this group has just holed up there and are just waiting to fight it out, maybe to the last." — Agence France-Presse