Five civilians who were used as human shields were missing, and soldiers were pursuing the assailants who had quickly retreated, military spokesman Brigadier General Restituto Padilla told reporters in Manila.

Padilla said the gunmen attacked a military outpost at daybreak in Pigkawayan, a farming town about 160 kilometres (100 miles) from Marawi city where fighters linked to the Islamic State (IS) group have been battling troops for a month.

The attack could be a diversionary tactic to ease pressure on the militants in Marawi, local police commander Chief Inspector Realan Mamon said on radio.

Padilla said the attackers belonged to the Bangsamoro Islamic Freedom Fighters (BIFF), one of four armed groups in the southern Philippine region of Mindanao that analysts say have pledged allegiance to IS.

He said the gunmen attacked the lightly defended outpost, then exchanged fire with troops.

“It’s already resolved. The enemy has withdrawn… they failed,” Padilla said, adding that troops were in pursuit of the militants.

Local authorities initially reported the gunmen had occupied a school before students arrived.

But Padilla made no mention of any incident at the school.

The area around Pigkawayan is made up of marshlands, mountains and farmlands.

Padilla said there were no confirmed casualties but the military had yet to locate the five people initially used as human shields.

Pigkawayan town mayor Eliseo Garsesa said about 200 gunmen were involved, while a police report said there were about 300.

Padilla said the BIFF, a small insurgent group believed to have just a few hundred fighters, had seemingly sought to capitalise on the military being focused on the Marawi war.

“By and large they are just taking advantage… of the situation that we have a very slightly defended outpost and that they think our forces are elsewhere in the province,” Padilla said.

– Hardline groups –

Muslim rebels have been fighting for more than four decades for an independent or autonomous region in the south of the mainly Catholic nation, with the conflict claiming more than 120,000 lives.

The major rebel organisations have signed, or are pursuing, peace deals with the government, but small hardline groups such as the BIFF have vowed to continue fighting.

Philippine President Rodrigo Duterte declared martial law across the southern region of Mindanao on May 23 immediately after fighters flying the IS flag rampaged through Marawi.

Their assault on Marawi ignited an unprecedented urban war that has claimed hundreds of lives and which Duterte has warned is part of an IS campaign to establish a base in Mindanao.

The fighting has left Marawi, the most important Muslim city in the Philippines, largely in ruins.

The militants involved in the Marawi fighting are mostly from the Maute and Abu Sayyaf organisations, which have united with the BIFF under the IS umbrella, according to the government.

The military has said foreign fighters, including those from Chechnya, Indonesia and Malaysia, have also joined the Marawi conflict.

IS has ambitions of setting up a caliphate in Southeast Asia — home to largely Muslim nations like Indonesia and Malaysia — as the group loses territory in Iraq and Syria.

Garsesa, the town mayor, said local authorities had received text messages over the past several days about an attack somewhere in the area.

The BIFF was blamed for attacking at least nine Mindanao towns in 2008, with the assaults claiming about 400 lives and forcing 600,000 people to flee their homes.