MANILA (Reuters) - Philippines President Rodrigo Duterte will ask Congress to extend martial law in the volatile southern island of Mindanao to quell an insurgency, cabinet officials said on Sunday.

Duterte placed the restive region of 22 million people under military rule on May 23 after Islamist militants took over parts of the southern Marawi City in what was the Philippines’ biggest security crisis in years. Martial law is due to expire on Dec. 31.

The Philippine leader will formally request on Monday a one-year extension of martial law, Executive Secretary Salvador Medialdea told reporters.

The 23-member Senate and the 296-member House of Representatives will vote once they convene in joint session. Lawmakers are due to go on recess on from Dec. 16 to Jan. 14, 2018.

Military rule should be extended in Mindanao given threats from Maoist guerrillas, Islamist militants and separatist groups, Presidential Communications Secretary Martin Andanar said.

Militants linked to Islamic State, which tried to gain a foothold in Southeast Asia by capturing parts of Marawi City, are strengthening their recruitment programs, Andanar said.

“There were intelligence reports saying they are planning to attack another city,” Andanar told a radio interview.

The request comes nearly two months after Duterte declared the liberation of Marawi City. More than 1,100 people - mostly militants - were killed and 350,000 displaced by the Marawi unrest.

Continuing martial law beyond the initial 60-day limit requires lawmakers’ approval, but the constitution does not restrict how long it can be extended.