An iron bar balanced between two metal poles is the only barrier between Cameroon and Nigeria in this dusty corner of the Sahel.

Children playing games skip around it. Inhabitants on either side greet each other and share jokes in the same language. The currencies of either country are accepted in markets as worried residents stock up on supplies.

As the threat of Nigeria's Boko Haram militant group increasingly looms over Cameroon, its government has dispatched over 1,000 troops and heavy armour to the north.

But the scene in Amchide, a small village on the Cameroon side, shows how hard it is to police a remote area where militants have deep ties with communities on both sides of the border and can slip from one to the other unnoticed.

"We are living here in total fear because you don't know if your next-door neighbour is Boko Haram. And we don't know what can happen to us at any time," said Samson Niba, who lives in Maroua, the main town in Cameroon's Far North region.

"The problem with our brothers up north here is that they are too secretive. They can host a Boko Haram suspect and they will not alert anyone even if they know what he is," he added.

Boko Haram has killed thousands in Nigeria since 2009, when it began a campaign of shootings and bombings in a bid to create an Islamist state. The group gained global notoriety in April when it kidnapped more than 200 schoolgirls from a village in Borno state in northeastern Nigeria, adjoining Cameroon.

The militants have also become a regional threat and West African leaders last month pledged to wage "total war" on the group, which has kidnapped foreigners in Cameroon and has been linked to a series of plots in Niger.

A bullet-riddled and burned-out four-wheel-drive vehicle sits abandoned just on the Nigerian side of the Amchide barrier, destroyed by Cameroon security forces as militants tried to cross over from Nigeria, according to residents.

Following a tip-off from locals, Cameroonian security forces on Monday arrested 40 suspected militants, including 20 picked up from the market, in Maroua, Colonel Felix Nji Formekong, the deputy regional commander, told Reuters.

"This is a particularly sensitive zone, especially on local market days ... which are often used as an opportunity by Boko Haram to infiltrate our land," said Formekong.

"Once they (the militants) are among the people it is difficult to identify them," he said.

WAR DECLARED

Cameroon's move to bolster its northern defences comes amid stinging accusations from Nigeria it has not been doing enough to counter the threat.

But a similar military deployment by Nigeria in its northeast has failed to stop Boko Haram attacking villages there almost daily and planting bombs in the capital Abuja.

A string of foreigners have been kidnapped in Cameroon's north over the last year. Among them were a French family as well as two Italian priests and a Canadian nun, who were taken into Nigeria before their eventual release.

In another incident, suspected Boko Haram rebels attacked a Chinese work site in northwest Cameroon in May, killing at least one soldier and abducting 10 Chinese workers whose fate is unclear.

Cameroon officials reject accusations they are not doing enough and say intelligence cooperation with Nigeria is good.

Formekong said Cameroonian forces had killed at least 10 militants in the north since Sunday. State media say government troops have killed dozens of others in recent weeks. The information has not been independently verified.

"Cameroon has ... declared war against Boko Haram and there can be no doubt that real actions are being taken on the ground," Lieutenant-Colonel Didier Badjeck, an army spokesman, said while visiting the north with journalists this month.

MIXED RESULTS

Some residents in Amchide, where soldiers crouch in trenches or keep watch from behind sandbags at positions backed by armoured cars, welcome the heavy deployment.

"Now calm has returned to this village, we can go on with our activities normally, live and sleep in peace," said resident Brah Omara.

Formekong said the military was cooperating with residents, many of whom were returning from hiding in the bush.

In a bid to win hearts and minds, troops in Ndiguina, a village just east of the frontier, handed bags of rice, cooking oil and sugar to residents.

Some locals, however, complain restrictions that include an overnight curfew, as well as the militant threat, are making it harder than ever eke out a living in a region cut off by bad roads and is one of Cameroon's poorest.

Hostage-taking has scared off potential tourists. Ze Bita, head of customs in the Far North region, said there had been a sharp drop in customs revenues, even though the security presence has cut down on cotton smuggling into northern Nigeria.

"I am afraid of Boko Haram. The whole population is afraid," said local woman Hadama Madina. "The government has even chased us away from this market because of Boko Haram."