As one of Cameroon's top military commanders vowed to wipe out Boko Haram militants 'once and for all' in the country, vigilante groups have emerged to try to protect civilians from the brutal jihadis.

A small band of locals have been arming themselves with homemade bows and arrows and ancient guns to watch out for the growing threat of Boko Haram suicide bombers.

Reuters's Joe Penney visited the town of Kerawa in northern Cameroon, where the local volunteer defence force have been speaking about their fears of female suicide bombers and jihadis hiding among refugees.

'We're here to look out for suicide bombers,' said Adama Simila, a member of a local civilian defence force in the town of Kerawa.

The 31-year-old's only weapon to protect him from Boko Haram is a knife which he keeps close to his side while on patrol with the army.

The group have been given permission by their local government to bear arms and have the power to carry out intelligence gathering missions, question travellers, and denounce to the military anyone deemed suspect.

Last week they intercepted two female suicide bombers and handed them over to the army before they were able to detonate.

'We are not 100 percent dependent on this information, but this information is crucial,' said Lieutenant-Colonel Tetcha, who is not only defending Cameroon but also a growing number of Nigerians.

Young girls have been taken by the jihadi group and used as sex slaves before being forced to carry out suicide bomb attacks as part of a cruel tactic increasingly used by the struggling extremists.

A regional offensive last year drove the insurgents from most of their traditional strongholds, denying them their dream of an Islamic emirate in northeastern Nigeria.

An 8,700-strong regional force of troops from Benin, Cameroon, Chad, Niger, and Nigeria is seeking to finish the job but now Boko Haram have been using guerrilla tactics to target civilians.

'I'm not scared. They are people, we are also people. We must die to live,' said Simila, who was at the Kerawa market in September when two girls detonated themselves, killing 19 people and injuring 143 others. A nearly identical bombing at the same market followed in January.

Outside Nigeria, Cameroon has been hardest hit by Boko Haram, which now operates out of bases in the Mandara Mountains, Sambisa Forest and Lake Chad.

Since August 2014, the jihadi group has carried out 336 attacks in Cameroon, according to the Cameroonian army, which has lost 57 of its own men while defending the north.

Of 34 recorded suicide bombings killing 174 people, 80 percent were carried out by girls and young women aged 14 to 24 years.

Girls abused as sex slaves by the group are psychologically damaged and therefore more vulnerable, the army says. Boko Haram also uses girls because they are thought less likely to arouse suspicion, although that may be changing now.

'The goal now is to stop Boko Haram incursions into villages, stop them from planting IEDs (home-made bombs), and stop suicide bombings,' said Lieutenant-Colonel Felix Tetcha, a senior officer in the army's operation against Boko Haram.

Cameroon have deployed 10,000 troops in the north to protect civilians from Boko Haram. The army's Rapid Intervention Brigade (BIR), comprised of its most professional, best equipped soldiers, patrols a high-risk 400-km (250-mile) stretch of the border with Nigeria, according to Reuters.

The U.S. military has provided the BIR with vital equipment, intelligence and training while a small American military camp has reportedly been seen inside another BIR base in nearby Maroua.

Still, the terrain is mountainous and Boko Haram has rigged many roads with explosives designed to kill soldiers. Army officers are convinced that some fighters from Boko Haram, which pledged allegiance to Islamic State last year, have been trained at ISIS camps in Libya.

Armed incursions by Boko Haram fighters have dropped. But the army does not have enough soldiers to deploy in every town in northern Cameroon, and suicide bombers strike regularly, often several times in a single week.

'The border is under control, but it's still very porous,' said Lieutenant-Colonel Emile Nlaté Ebalé, head of operations and logistics for the BIR's mission in the north.

Close to the border sits the U.N.-run Minawao camp, home to nearly 57,000 refugees who have fled Boko Haram in Nigeria.

'Everybody suffers in this place,' said James Zapania, a 24-year-old camp resident from Gwoza, Nigeria. 'We're not worried about Boko Haram coming here, we're worried about food.'

Refugees like Zapania often receive a chilly welcome from suspicious local villagers, many of whom view them as collaborators or even underground Boko Haram fighters.

According to one Cameroonian officer, the army has removed a number of individuals from Minawao for 'activities that were not in line with the behaviour of a normal refugee'.

Suspicion is everywhere. And while Boko Haram infiltrators make up only a tiny portion of fleeing refugees, many, including the Cameroonian military, fear that desperation provides fertile ground for recruitment.

'We need to act quickly. There are young people with no work who could be vulnerable. When people are hungry, they are easily approached,' said Colonel Didier Badjeck, a Cameroonian military spokesman.