(CNN) The Philippines government has voted to extend martial law in the southern island of Mindanao, following a year in which Islamic militants shocked the nation in taking -- and holding, for several months -- pockets of a Muslim-majority city there.

State media PNA reported that a joint session of Congress approved, 240-27, President Rodrigo Duterte's request to extend military rule on the island, which has a sizable Muslim population, until the end of 2018.

The current period of martial law, first enacted when ISIS-afflilated Islamist groups stormed the city of Marawi in May and since extended, was due to expire at the end of the year.

Duterte declared Marawi liberated in October, but many in the Philippines and abroad were shocked by how long the rebels had been able to hold the territory.

The initial period of martial law was extended in July . At the time, there were concerns that the extension contravened the country's constitution.

Duterte had requested the extension in writing Sunday. In the letter, he cited homegrown and foreign terror attacks, alongside the "intensified" rebellion of the communist New People's Army (NPA), as reasons for the extension, according to CNN affiliate CNN Philippines.

He added that his defense secretary, Delfin Lorenzana, also recommended the year-long extension.

Militants beaten back

Congress' extension comes despite government forces eventually reclaiming Marawi.

Duterte declared the liberation of the city after finally defeating a coalition of ISIS-sympathetic terror groups who held the city in a five-month siege. The occupation was brought to an erupt end after the death of the militants' leaders, Omar Maute and Ipsilon Hapilon in a firefight.

In the days and weeks following the invasion, hundreds of thousands of people fled the city to escape the militants' clutches.

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ISIS had had aspirations for Mindanao as a potential location for ISIS' wadiyah, or state, in Asia, appointing Hapilon, the leader of local militant group Abu Sayyaf as the region's emir in 2014.

When he first declared martial law in Mindanao in May, Duterte had warned the order could last up to a year.

"If it would take a year to do it, then we'll do it," he said at the time. "If it's over within a month, then I'd be happy."

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Wary populace

The imposition of martial law in the Philippines has long been a point of contention in the Philippines.

The country suffered 14 years of martial law under former dictator Ferdinand Marcos, who was ousted in a peaceful revolution in 1986. The possibility of martial law being extended throughout the country has stoked fears among some Filipinos of a repeat of that period.

During the 14 years, perceived opponents of Marcos were subjected to numerous human rights violations, including imprisonment, forced disappearances and torture. Extrajudicial killings carried out by Marcos' secret police were also common.

However, many young Filipinos today have little or no knowledge of the period under Marcos, and of life under martial law.