MARAWI, PHILIPPINES (AFP) - The Philippine military said Sunday (Sept 17) it had captured the command centre of Islamic State in Iraq and Syria (ISIS) group supporters who have besieged a southern city for nearly four months.

Security forces have engaged in ferocious street to street combat and launched airstrikes in their efforts to expel the fighters from the city of Marawi, in a conflict that has raised fears that ISIS is looking to establish a South-east Asian base in the Philippines.

The military said it had captured the militants' control centre in a deadly battle that began Saturday in a mosque and another building.

"This enormous (military) gain further weakened the terrorist group by denying them their erstwhile command and control hub," military chief General Eduardo Ano said in a statement.

"As follow up and clearing operations continue, we expect the enemy to yield more previously occupied positions, but not without a fight," he said. "We are ready for that."

Hundreds of armed extremists flying the black flag of the ISIS movement in the Middle East occupied Marawi, the Islamic capital of the mainly Catholic Philippines, on May 23.

More than 800 militants, government troops and civilians have since been killed in the conflict, which has forced thousands to flee their homes and destroyed large parts of the once-bustling city.

President Rodrigo Duterte has deployed thousands of troops and imposed martial law across the southern third of the country to deal with the crisis, while the military has launched a US-backed air campaign against the militants.