Duterte declared martial law on May 23 following a bloody siege of the southern city of Marawi by Islamic State group- aligned militants, the most serious security crisis he has faced since assuming power in June last year.
In a letter to the Senate and the House of Representatives read by his spokesman Ernesto Abella, Duterte said that after consulting security officials, he has concluded that the rebellion in the south will not be quelled completely by July 22.
Under the constitution, in case of invasion or rebellion, when public safety requires it, the president can declare martial law for no more than 60 days. He can ask Congress to extend such a proclamation to a period of time to be determined by the lawmakers.
More than 550 people, including 413 militants, have been killed in nearly two months of fighting in Marawi, a bastion of Islamic faith in the south.
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US Ambassador Sung Y Kim said the United States will be sending two surveillance aircraft to help Philippine forces battling the militants in Marawi.
"We're deeply concerned about the security situation in Marawi," Kim said. The US has been providing munition, intelligence and surveillance equipment and special operations troops to advise the Philippine military in the south. It will also assist in rebuilding Marawi, he added.
Duterte met with lawmakers late yesterday. He called for a special session of Congress, which is on break, on Saturday to tackle the proposed extension of martial law.
Congress opens its new regular session on July 24, when Duterte delivers his State of the Nation address.
"It is totally not justified" and "just a whimsical misuse of power," said Sen. Antonio Trillanes IV, adding that the military says the number of militants was down to 60.
"I have already forewarned the public of Duterte's authoritarian tendencies and this is further proof of it," he added.
Rep Tom Villarin of the left-wing political party Akbayan said it was "a whimsical proposal that has no substantive grounds other than the president's wishes.