MANILA (Reuters) - Five Philippine presidential guard members were wounded and a paramilitary guard killed on Wednesday in an attack by Maoist rebels disguised as soldiers, security officials said, an incident that could dent a stuttering peace process.

The attack on the elite Presidential Security Group (PSG) in North Cotabato province could complicate efforts by the government and the National Democratic Front, the communist rebels’ political arm, to find a lasting solution to end a half-century conflict that has killed 40,000 people.

President Rodrigo Duterte was not in the region on the southern island of Mindanao when the clash between the guards and the New People’s Army (NPA) rebels erupted.

“Five of our troops in two vehicles were slightly wounded in the firefight,” PSG commander Brigadier-General Louie Dagoy told reporters.

Police said a paramilitary guard was killed and one was wounded. Both were traveling with the PSG unit.

The peace process, an important initiative of Duterte, has been fraught with breakdowns, with both sides abandoning unilateral ceasefires in February, blaming each other for launching attacks while talks were going on.

Informal talks in the Netherlands, where the exiled leader of the communists is based, are due on the weekend, aimed at restarting the stalled process.

The Philippines has been beset by separate communist and Muslim insurgencies in various parts of the country for decades. Communist and Muslim rebels both operate on Mindanao.

The clash at a checkpoint came a day after Duterte asked Congress to extend martial law until the end of the year to tackle rising Islamist militancy.

Duterte on Tuesday met his negotiators dealing with the communists, and told them not to agree to a bilateral ceasefire until the NPA ceased attacks on troops. He also called the political leadership to keep its fighters under control.

“We will bring this incident to their attention,” Duterte’s top peace adviser, Jesus Dureza, said in a television interview.

“The reality is there is no binding ceasefire agreement right now, that’s why these incidents continue to happen.”

The violence highlights the challenge facing a military stretched on multiple fronts in Mindanao, an island of 22 million people.

Government forces are battling to defeat Islamic State-inspired rebels who have occupied the heart of Marawi City for 58 days, while operations continue on islands to the west of Mindanao against Abu Sayyaf rebels behind kidnappings and piracy.

Military chief Eduardo Ano said Maoist rebels were exploiting the security crisis.

“That’s why you are seeing now an increase in the intensity of encounters nationwide,” he said.

Documents obtained by the army showed Maoist rebel leaders had ordered guerrillas to step up attacks after martial law was imposed in Mindanao on May 23, Ano said.