MANILA • Philippine President Rodrigo Duterte has threatened to jail critics of his use of martial law in the violence-wracked south, days before the Supreme Court is set to rule on its legality this week.

Mr Duterte declared military rule across the region of Mindanao, home to about 20 million people, in late May to quell what he said was a fast-growing threat from the Islamic State in Iraq and Syria group there.

The Philippine leader has insisted he would ignore the findings of the court, which has constitutional oversight, vowing only to listen to recommendations from the armed forces.

"It's not dependent on the whim of the Supreme Court. Should I believe them? When I see the situation is still chaotic and you ask me to lift it? I will arrest you and put you behind bars," Mr Duterte said in a speech before local officials last Saturday. "We can talk of anything else and make compromises maybe, but not when the interest of my country is at stake."

Government forces are continuing to battle militants occupying the city of Marawi, with aerial bombardment and ferocious street-to-street combat that have left some 400 people dead and forced nearly 400,000 people in the wider area to flee their homes.

Yesterday, the Philippine army announced that the Philippines and Indonesia will jointly patrol the Celebes Sea this week to stop Islamist militants reaching Mindanao and Marawi. The joint patrol is aimed at strengthening border security and improving interoperability, military spokesman Ezra Balagtey said in a statement.

The two countries' warships will sail from the Philippines' Davao City on Thursday. "The coordinated patrol... is intended to strengthen the security of the Davao Gulf and the common boundary of the two countries in the southern archipelago, particularly along the Celebes Sea," said Mr Balagtey.

Mr Duterte has faced a backlash from opposition lawmakers, who last month asked the Supreme Court to reject the declaration of martial law, which they have slammed as unconstitutional.

The 1987 Constitution imposes limits on martial law to prevent a repeat of the abuses under dictator Ferdinand Marcos, who was deposed by a famous "People Power" revolution the previous year.

The charter allows the Supreme Court to review the factual basis for proclaiming martial law, which is limited to an initial period of 60 days. If a president decides to extend martial law, Congress can review and revoke it.

AGENCE FRANCE-PRESSE, REUTERS