Duterte
Philippine President Rodrigo Duterte Reuters

Philippine President Rodrigo Duterte has offered large bounties on Monday for the militant chiefs, who are holding parts of a southern city, as the military warned that defeating the militants was proving "difficult" and might take longer time than expected.

Reports show that hundreds of gunmen loyal to the Islamic State in Iraq and Syria (ISIS) group are battling air strikes, artillery and ground forces in Marawi. The government says that nearly 2,000 civilians are believed to have been trapped for two weeks in districts held by the militants.

According to government officials, the fighting has killed at least 178 people and caused nearly a quarter of a million others to flee.

A military statement said that fresh rewards totalling 20 million pesos (US$410,000) are on offer from Duterte for the "neutralisation" of Isnilon Hapilon, Abdullah Maute and his brother Omarkhayam Maute.

"We hope that this will bear significant accomplishments leading to the eventual arrest and neutralisation of Isnilon Hapilon and the Maute brothers," General Eduardo Ano, the military chief of staff, said in the statement. Ano described Hapilon as the "amir" or top leader of ISIS in the Philippines and said that half of the amount will be for him.

The US government has also offered a US$5 million bounty on Hapilon's head for the kidnapping and killing of US citizens, as well as a standing bounty of 7.4 million pesos by the Philippine government.

Manila says that it is fighting up to 250 gunmen holed up in central Marawi, which is a largely Catholic nation's most important Islamic city and home to 200,000 people. The gunmen are a combination of Hapilon's Abu Sayyaf group. It has been blamed for the country's deadliest bombings as well as kidnappings, and militants led by the Mautes and based in the Marawi region.

On Monday, the provincial government said that more than 224,000 residents of the city and nearby towns have fled the fighting.

Following the violence that erupted in the city, Duterte imposed martial law across the southern region of Mindanao, home to 20 million people, to quell what he said was an ISIS bid to establish a base in the country.

The military warned on Monday that defeating the militants, who are also believed to include several foreign fighters, was proving "difficult" even though 120 of them had been killed.

The military spokesman Lieutenant-Colonel Jo-ar Herrera said the gunmen were using an extensive network of tunnels and basements built many years ago beneath the buildings they have seized. He also said that they had also sited powerful weapons such as machine guns inside mosques and traditional Islamic schools that are off-limits to air strikes and artillery attacks.

"What they are doing is un-Islamic," he told AFP.

Herrera added that the buildings are also believed to contain at least a month's worth of food stocked up by their owners for the entire Muslim fasting month of Ramadan that coincided with the fighting.