World Bulletin / News Desk

Philippine President Rodrigo Duterte asked Congress on Monday to extend martial law across the southern third of the country until the end of next year to combat Islamist militants and communist rebels.

But in a letter to lawmakers released by his office, Duterte said the extension was necessary in the southern region of Mindanao to contain an ongoing rebellion by supporters of the Islamic State group as well as a rising threat from communist guerrillas.

"I ask the Congress of the Philippines to further extend the proclamation of Martial Law and the suspension of the privilege of the writ of habeas corpus in the whole of Mindanao for a period of one year" from January 1, the letter said.

Duterte initially imposed military rule across Mindanao -- home to about 20 million people -- in May to quell an uprising by IS militants in Marawi city.

Hundreds of gunmen rampaged through the Islamic city of Marawi in what authorities said was part of an attempt to establish a Southeast Asian base for IS in the mainly Catholic Philippines.

A US-backed military campaign took five months to defeat the militants, with the battle claiming more than 1,100 lives and leaving large parts of Marawi in ruins.

Duterte declared in October that the city was "liberated" and military chiefs said most militant leaders, including the IS leader in Southeast Asia, had been killed. 

But authorities have continued to warn that those who escaped are regrouping and recruiting in Mindanao.

In his letter, Duterte said martial law had been key to containing the IS supporters in Marawi, which is about 800 kilometres (500 miles) south of Manila, the nation's capital.

"The remnants of their group have continued to rebuild their organisation through the recruitment and training of new members and fighters to carry on the rebellion," Duterte wrote.

He also warned that communist rebels, who are waging one of Asia's longest insurgencies, had stepped up their "terrorist acts" and wanted to overthrow the government.

The president last month called off peace talks with the communists, blaming continuing deadly attacks by the rebels.