The Philippine Congress approved a 12-month extension of martial law in the restive Mindanao region on Wednesday, after President Rodrigo Duterte argued for maintaining tough security measures to stop Muslim extremists from regrouping.

A joint legislative session voted 235-28 in favor of retaining military rule in Mindanao until the end of 2019, prolonging what was already the country's longest period of martial law since the 1970s.

Mindanao's mostly poor Muslim areas have for decades been troubled by banditry, piracy, and armed rebellions. However, May last year saw the eruption of the country's fiercest conflict since WWII, when an alliance of extremists seeking to create an Islamic State enclave attacked and held Marawi city through five months of government air strikes and ground offensives.

"Notwithstanding the substantial gains achieved during the martial law period, we cannot turn a blind eye to the reality that Mindanao is in the midst of rebellion," Duterte wrote in a letter to Congress.

Duterte's spokesman and the military thanked lawmakers after the vote, and said rights and civil liberties would be preserved under martial law intended to prevent radical groups from expanding beyond Mindanao.

Opposition lawmakers said the extension was unjustified because there was no longer a rebellion to quell.

Representative Edcel Lagman said what remained of Islamic State's allies in Mindanao were "quixotic and phantom fighters who are unable to revive a vanquished "rebellion" or launch a new one."

(Cover photo: Smoke billows from burning houses after as fighting between government troops and Islamist militants continues in Marawi on the southern island of Mindanao, June 26, 2017. /VCG Photo)