The latest bomb attack by suspected Islamist militants in Davao City is Philippine President Rodrigo Duterte’s biggest challenge since taking office in June, and his move to declare a nationwide “state of lawlessness” is rattling nerves.

At least 14 people were killed and dozens more injured in Friday’s blast at a busy city-center night market. As paramedics and doctors attempted to treat the survivors, Abu Rami, a spokesman for the extremist Abu Sayyaf group, called local media and claimed its sympathizers had orchestrated attack, saying it was a reprisal for military operations to hunt to down and eliminate the Islamic State-linked group on the island of Jolo. In the past week, 15 soldiers and at least 25 militants were killed in fierce fighting on Jolo.

With the Philippine army now preparing to deploy more troops to Jolo, the Abu Sayyaf spokesman warned of further attacks. Mr. Duterte subsequently declared a nationwide state of lawlessness.

The president’s declaration falls short of imposing martial law, but it does allow military personnel to be deployed to support police in setting up checkpoints, or in carrying out searches. Some legal experts suggest declaring a state of lawlessness is a way of informing the Philippine army that it might be called upon to perform a policing role.

It isn’t without precedent, either. Davao City and its neighboring areas were also placed under a state of lawlessness by then-President Gloria Macapagal Arroyo in 2003, after two bombings claimed 38 lives.

But some observers fear Mr. Duterte’s response might amplify the impact of Friday night’s attack. “The callous disregard shown by the attackers for people’s right to life must not be met by government action that itself disregards human rights,” Champa Patel, Amnesty International’s senior adviser for Southeast Asia, said in a statement.

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