President Rodrigo Duterte is expected this week to formally ask Congress to further extend the imposition of martial law in Mindanao for one year, official said on Sunday.

Executive Secretary Salvador Medialdea told reporters in a text message that Duterte's letter seeking for the extension of martial law will be transmitted to Congress on Monday.

"Letter will be out tomorrow," he said.

Earlier in the day, Presidential Spokesperson Harry Roque said Duterte's paramount concern is "the security of our people especially the Mindanaoans in the face of threats and the use of available means under the law to fight them."

Presidential Communications Operations Office Secretary Martin Andanar, in a radio interview, also explained the need to extend martial law.

He cited that the Daula Islamiya, partner of Maute and Abu Sayyaf groups, is now recruiting members in Mindanao.

Citing an intelligence report, he said this group is planning to attack another city in the country. He did not mention the city.

"So therefore, we need to closely monitor them and there is a threat from Bangsamoro Islamic Freedom Fighters in Lanao and Cotabato and the ASG in Basilan, Tawi-tawi and Sulu," he said.

"So therefore, we need to extend martial law in Mindanao. There is also a threat from CPP-NPA-NDF (Communist Party of the Philippines, New People's Army and National Democratic Front)," he added.

Andanar said there is still time for Congress to decide on the proposal to extend martial law.

Both the Philippine National Police and the Armed Forces of the Philippines, through the Department of National Defense, have recommended to the President the extension of martial law.

While the PNP has recommended for an extension of another year, the AFP was mum on its proposal.

The state security forces have recommended the extension of martial law due to the continued threats from terrorist groups in Mindanao.

Aside from the remnants of the Islamic State-inspired Maute terror group, Abu Sayyaf Group, and BIFF, the government forces are also pursuing the CPP-NPA, which Duterte recently declared as a terrorist group.

Duterte declared martial law in Mindanao last May 23 for 60 days shortly after the terrorists occupied and attacked Marawi City. But the martial law declaration was extended until December 31, 2017. Celerina Monte, Ella Dionisio/DMS