Marawi was taken over May 23 by insurgents from the Abu Sayyaf and Maute militant groups who were backed by foreign fighters. The seizure, which took place while Mr. Duterte and his top security officials were visiting Russia, prompted the president to cut short the trip and place the island of Mindanao — the second largest in the Philippines — under military rule.

He ordered Philippine troops and the air force to Marawi to try to dislodge the militants. The battle forced an estimated 200,000 residents to flee Marawi, the only predominantly Muslim city in the Philippines, a mostly Roman Catholic country.

Military officers leading the assault on the ground have said that the militants were entrenched in central Marawi, where they are believed to be holding out with enough firepower to withstand the military’s air bombardments and its ground assaults. The insurgents are also believed to be holding dozens of civilians captive, including a Catholic priest who had appealed to the government to stop its air assaults.

The military said on Monday that 56 days of fighting had killed at least 97 government troops and 45 civilians. At least 405 extremists had also died, including several foreign fighters, the government said, leaving behind about 60 rebels.