MANILA, Dec. 5 (Xinhua) -- Philippine President Rodrigo Duterte has approved "in principle" the recommendation of the security forces to place Philippines's southern Mindanao under the martial law for another year, the Philippine defense chief said on Wednesday.

The Philippine Defense Secretary Delfin Lorenzana has endorsed the recommendation of the Armed Forces of the Philippines (AFP) to extend martial law. The Philippine National Police (PNP) also endorsed the one-year extension.

Lorenzana said martial law was discussed on the sidelines of the cabinet meeting late on Tuesday.

However, Lorenzana said nothing is final until Duterte formally sends a letter to Congress to ask for an extension.

Presidential Spokesperson Salvador Panelo on Wednesday said Duterte "is still studying" the recommendation of the security forces.

Local terrorists belonging to the Maute Group and Abu Sayyaf Group attacked Marawi City on May 23, 2017, prompting Duterte to place the entire Mindanao under martial law for 60 days.

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

In October 2017, the government finally gained control of Marawi. More than 1,200 people were killed in the battle to retake the city that fell into ruin.

A year after Marawi was retaken, the government has yet to implement the recovery, reconstruction and rehabilitation of the southern city.

The Duterte government argued that martial law is needed to ensure peace and order in the Mindanao region while the rebuilding of Marawi is underway.