FILE PHOTO: Activists display a banner and a mural of President Rodrigo Duterte to denounce the declaration of martial law in Mindanao island, one year after Islamic militants occupied parts of the southern Philippine city of Marawi during a protest outside the presidential palace in Metro Manila, Philippines May 23, 2018. REUTERS/Romeo Ranoco/File Photo

MANILA (Reuters) - 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 brutal 1970s era of late dictator Ferdinand Marcos.

Mindanao’s mostly poor Muslim areas have for decades been troubled by banditry, piracy and armed rebellions by separatist and communist militias, some of which have been managed by truces and decentralization moves.

However, May last year saw the eruption of the country’s fiercest conflict since World War Two, 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.

“It makes me wonder, is this the new normal?” Senator Franklin Drilon told the session.

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.”