Philippine President Rodrigo Duterte has asked congress to extend martial law in the southern Mindanao region until the end of the year, to allow armed forces to quell the threat from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

Duterte declared military rule over Mindanao, an island of 22 million people, for 60 days on May 23 when hundreds of ISIL-linked fighters occupied parts of Marawi city.

ISIL's presence led to clashes that have killed more than 500 people.

But with scores of armed fighters holding out against government forces, Duterte met politicians late Monday and asked them to extend the law when it lapses on July 22, his spokesman Ernesto Abella said.

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"There is a looming situation in Mindanao which needs to be totally and completely addressed," Abella said on Tuesday, adding that the primary objective of the extension was "to allow our forces to continue with their operation, unhampered by deadlines."

The country's constitution allows the president to impose martial law for up to 60 days.

Beyond two months, the president can extend the rule for a period to be determined by congress, Abella said.

With Duterte's allies dominating congress, House of Representatives speaker Pantaleon Alvarez told the AFP news agency he saw no obstacle to approving the president's request.

Congress will convene on Saturday to discuss the extension.

Senator Sherwin Gatchalian, who attended Monday's meeting with Duterte, said: "He also explained clearly his fear that terrorism might slowly spread throughout Mindanao and eventually the country."

Duterte told the politicians that 600 buildings had yet to be cleared of bombs or armed men, Senator Joseph Victor Ejercito told AFP.

Security forces have been conducting a US-backed offensive to root out the gunmen, using air attacks and artillery fire.

Al Jazeera's Yaara Bou Melhem, reporting from Manila, said the Philippine authorities are more accustomed to jungle warfare as opposed to urban guerilla warfare.

"The fact that after two months of fighting, the Philippine authorities have not been able to retake parts of Marawi city just goes to show how unprepared they were for this firefight," she said.

"These fighters seem to have quiet a bit of firepower in order to keep this city for as long as they have."

Also on Monday, government and rebel representatives submitted to Duterte a new draft law, which aims to establish a more powerful Muslim autonomous region in the country's south under a 2014 peace deal that stalled in congress under his predecessor.