MANILA, Dec. 11 (Xinhua) -- Philippine President Rodrigo Duterte formally requested the congress on Monday to extend martial law in the entire Mindanao until Dec. 31, 2018 to quell the threat of Islamist terrorism in the southern Philippine region.

In a letter addressed to Congress, Duterte said, "a further extension of the implementation of martial law and suspension of the privilege of the writ of habeas corpus in Mindanao will help (the government security forces) quell completely and put an end to the ongoing rebellion in Mindanao."

Duterte said placing Mindanao under martial law for another year will also prevent violent extremism from spreading to other parts of the country.

"Public safety indubitably requires such further extension, not only for the sake of security and public order, but more importantly to enable the government and the people of Mindanao to pursue the bigger task of rehabilitation and the promotion of a stable socioeconomic growth and development," Duterte said in the letter released on Monday.

Duterte said he based his decision on the recommendation of Defense Secretary Delfin Lorenzana, also the martial law administrator.

Citing the security assessment of the Armed Forces of the Philippines, Lorenzana said extending martial law in Mindanao for another year will ensure total eradication of Islamic State-inspired Da'awaful Islamiyah Waliyatul Masriq (DAWM), other like-minded local and foreign terrorist groups and armed lawless groups, and the communist rebels and their coddlers, supporters, and financiers.

Moreover, Duterte said IS-linked militants "have been monitored" to conduct "radicalization" through active recruitment, financial and logistical buildup and consolidation in central Mindanao particularly in Maguindanao and North Cotabato provinces, and Sulu and Basilan, the remote island provinces off Mindanao and known hideout of militants.

He noted that other IS-inspired groups Bangsamoro Islamic Freedom Fighters (BIFF) and the Abu Sayyaf Group (ASG) are also planning to sow terror in the impoverished region.

Lorenzana and security officials are scheduled to brief the senators on Tuesday on the Duterte's request for further martial extension.

IS-linked militants "Maute group" attacked Marawi City on May 23, prompting Duterte to place the entire Mindanao under martial law for 60 days.

Before the declaration lapsed on July 22, Duterte requested the congress to extend its effectivity until Dec. 31, 2017.

The government finally gained control of the city in October. More than 1,200 people were killed in the battle to retake the city, reducing the city into rubbles.

The 1987 Constitution only allows the president to declare martial law for a maximum of 60 days. Any extension would require approval from Congress.