Duterte was on a trip to Russian Federation when hundreds of militants went on a rampage in Marawi City, 800 kilometers south of Manila, on May 23, after government forces attempted to arrest a local leader of the Islamic State terrorist movement.
Addressing soldiers in the city of Butuan, north-east of Marawi, Mr Duterte said if the conflict escalated or spread, he would take unilateral action. "They have corrupted the name of God in the form of religion to kill many innocent people, for nothing".
The militants remain holed up in pockets of Marawi alongside hundreds of trapped civilians being used as hostages or human shields.
Senator Leila De Lima, the former justice minister and Duterte critic who was imprisoned in February on charges of drug trafficking, said the government should promptly inform the public about the 72-year-old President's condition in order to stem rumors and speculation, calling for a full disclosure as "a matter of public interest and national security".
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He missed a scheduled appearance the following day at annual Independence Day celebrations in Manila.
Presidential spokesman Ernesto Abella said Duterte was back home in the southern city of Davao to spend time with his family.
One of Duterte's aides later released photos which he said were taken on Thursday evening, showing the president standing up and looking healthy.
Abella said: "I'm not privy to those matters but I'm sure he's checked with his own experts".
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However, the President said in remarks to soldiers he visited late Saturday, he would not hesitate to issue a new martial law proclamation if the situation deteriorates and troops are needed to quell the extremists led by the Maute Group and Abu Sayyaf Group's sub-leader Isnilon Hapilon, tagged the emir for Southeast Asia of the Islamic State of Iraq and Syria (ISIS).
Mr. Abella said Mr. Duterte was taking time off because of a punishing schedule, which included regular visits to military camps and hospitals to support troops.
Mr. Duterte had repeatedly denied during last year's presidential election campaign that he suffered from cancer.
Duterte said then his doctor made him stop using it on learning he was "abusing the drug" by using more than the prescribed amount.
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Under the Constitution, the separately elected vice-president shall act as president if the incumbent dies, is permanently disabled or removed from office.