President Duterte imposed military rule for 60 days in the Mindanao region, home to 20 million people, on May 23 when gunmen waving black IS flags occupied Marawi city, triggering clashes that have killed more than 500 people.
But with scores of militants holding out against government forces, Duterte met with lawmakers late Monday and asked them to extend the law when it lapses on Saturday, spokesman Ernesto Abella said.
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The country's constitution allows the president to impose martial law for up to 60 days, enabling him to "call out the armed forces to prevent or suppress lawless violence, invasion or rebellion".
Beyond two months, the president can extend it "for a period to be determined by the Congress".
Duterte's allies dominate Congress, and House of Representatives Speaker Pantaleon Alvarez said Tuesday he saw no obstacle to approving the president's request.
In May, Duterte said he had made the move to stamp out an attempt by militants, including foreign fighters, to establish an IS caliphate on Philippine territory.
"He also explained clearly his fear that terrorism might slowly spread throughout Mindanao and eventually the country," Senator Sherwin Gatchalian, who attended the meeting with Duterte on Monday, told AFP.
Duterte told the legislators 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 strikes and artillery fire.
On Tuesday, US Ambassador to Manila Sung Kim said his government would give the Philippine military two Cessna planes to be used in Marawi.
"We are deeply concerned about the security situation in Marawi," Kim told ABS-CBN television.
"The Marawi situation is clearly a very difficult situation for the Philippines so we are going to do everything possible to support the [armed forces]."
