Philippine security forces warned on Tuesday of attacks in the restive south by a group of "bombers-for-hire" trained by Islamic militants.

Local insurgents in the south of the largely Catholic country have sharpened their detonating skills but a military cull of their leadership has left them without a gameplan and encouraged them to start offering their expertise for a fee.

"We've heard of a group of 'bombers-for-hire' because many of the people trained to assemble crude bombs were getting bored and were eager to make use of their talents," Major-General Raymundo Ferrer, an army division commander in the southern province of Cotabato, said.

Ferrer said it was too early to say if freelance bombers were behind a spate of attacks on bus terminals in the Cotabato area, including an explosion on Monday evening that wounded six people.

The Philippine government, with assistance from the United States, has been trying to wipe out Abu Sayyaf, a small but deadly local Muslim terror group, as well as renegade members of the Moro Islamic Liberation Front (MILF), a 11,000 strong separatist movement that has a truce with Manila.

A handful of members of Jemaah Islamiah (JI), a regional militant network, who are sheltering on the Philippines' remote southern islands are suspected to have trained Abu Sayyaf and other militants in bomb-making.

In a recent report, the International Crisis Group thinktank said that jihadists had allowed the "diffusion of bombing technology into the criminal underworld" in the southern Philippines.

Ferrer said they were getting reports that jihadists continue to conduct training with local Muslim rebels in some MILF bases on the main south island of Mindanao.

"There was even information shared by the police that crude bombs assembled in MILF bases were being moved closer to urban centres across Mindanao," he said.

Relations between the MILF and the Philippine government have deteriorated due to stalled talks over a homeland for Muslims in the south and Ferrer said he expected some isolated skirmishes to force Manila to resume negotiations.

"We expect them to create some noise and pressure government to resume talks as soon as possible."

The MILF has repeatedly denied any ties to JI, Abu Sayyaf or any other militant group.