Philippine President Rodrigo Duterte has asked Congress to extend martial law in the country's south by another year amid continuing concerns over possible militant attacks, although democracy advocates fear it could worsen human rights conditions.

Presidential spokesman Salvador Panelo said Friday that ending martial law could undermine progress by government forces in quelling insurgents "and may even strengthen the rebellion and propel it to other parts of the country."

Defense Secretary Delfin Lorenzana said he, along the military and police, backed the extension of martial law in the country's southern third, because Muslim extremists, including five to 10 foreign fighters, continue to plot bombings and other attacks.

Duterte placed the southern Mindanao region under martial law after hundreds of Islamic State group-linked militants besieged the Islamic city of Marawi on May 23, 2017, in the worst security crisis he has faced.

After five months, Philippine troops quelled the siege, which left more than 1,100 combatants and civilians dead and displaced hundreds of thousands of people.

The government has successfully asked Congress twice to allow an extension of martial rule in the south, the homeland of minority Muslims and site of decades-old Muslim separatist insurrections.

"With martial law in place, we have achieved substantial progress in addressing the rebellion in Mindanao," Panelo said. "A halt may only frustrate the progress we are witnessing in Mindanao and may even strengthen the rebellion and propel it to other parts of the country."

Opponents argue that extending martial law is unconstitutional because it's an "extreme measure" that can only be imposed when an actual rebellion against the government exists. They expressed concern that the move could be a prelude for Duterte to declare martial law throughout the Philippines.

The left-wing Karapatan group said a martial law extension would be a human rights "nightmare," citing claims of dozens of questionable killings of civilians by troops, displacement of villagers by military bombings and illegal arrests under martial law in the south. The military has denied such claims.

Filipinos remain hypersensitive to threats to democracy and civil liberties after they ousted dictator Ferdinand Marcos in a 1986 "people power" revolt that became a harbinger of change in authoritarian regimes worldwide. Marcos declared martial law in the Philippines in 1972 in a period marked by massive human rights abuses.

Concerns over Duterte's martial law have been sparked in part by his perceived authoritarian bent and the killings of thousands of suspects in a crackdown on illegal drugs that he launched after taking office in 2016 which has alarmed Western governments and human rights groups.