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"I may decide to expand the area to include Visayas because it is a walking distance actually," Xinhua news agency quoted Duterte as saying at the airport after cutting short his official visit to Russia.
He might also decide to suspend the writ of habeas corpus in the Visayas region, the President said, adding: "Remember this is not intended for law-abiding citizens."
Duterte said his martial law declaration for Mindanao, in the country's restive south, was necessary to fight militants there, CNN reported.
"It is our constitutional duty to enforce the law and provide security."
Duterte's order -- which covers Marawi city and the wider island of Mindanao -- came after deadly clashes broke out between Filipino government troops and Islamist militants on Tuesday, the CNN report said.