Manila (AFP) - Four of Philippine President Rodrigo Duterte's bodyguards were shot and wounded on Wednesday by suspected communist rebels a day after the insurgents warned of attacks, authorities said.

Duterte was not in the convoy when gunmen opened fire on two Presidential Security Group vehicles along a highway on the main southern island of Mindanao, they added.

A military official blamed the New People's Army -- the 4,000-member armed wing of the Communist Party of the Philippines -- for the ambush.

"This is part of their nationwide call for armed groups to oppose martial law by launching intensified offensives against government forces," Brigadier General Gilberto Gapay, a senior Mindanao military official, told radio station DZBB in Manila.

The ambush came a day after Duterte asked Congress for authorisation to place Mindanao under martial law until December to defeat Islamic State group-styled militants fighting security forces in the city of Marawi.

A 60-day martial rule is already in place over the island after the militants attacked Marawi on May 23, but Duterte conceded Tuesday he needed more time to defeat the IS-styled gunmen.

The communist party, which has been waging Asia's longest insurgency, called on its armed wing on Tuesday to launch offensives in response to Duterte's extended martial law plan.

The wounded soldiers were on their way to another Mindanao city when they ran into a checkpoint manned by guerrillas, their commander Louie Dagoy told reporters.

"Our troops were able to fire back and they called for help from a nearby militia base," a regional military spokesman, Major Ezra Balagtey, told AFP.

The communist insurgency that began in 1968 has claimed an estimated 30,000 lives, according to the military.

The rebels have been in on-and-off peace talks with the government since Duterte, a self-described socialist, was elected last year.

They were set to resume formal negotiations next month.

On Tuesday Duterte denounced the rebels for their alleged treachery in killing soldiers.

Saying the rebels were targeting soldiers who are not allowed to carry their assault rifles when they went off-duty, Duterte announced he was providing all of them with pistols for self-defence.

"The majority of the people have repudiated communism with all its brutality, and (they have) nothing really to offer to the country. It's almost bankrupt," Duterte added.