Cameroon, stepping up its cooperation with Nigeria against the deadly Islamist campaign by Boko Haram insurgents, is working to rout them from a key stronghold in a remote northern border region.

Amchide, a city in the far north of Cameroon near the Nigerian town of Banki, has become a hotbed for fighters from the extremist group -- one police officer estimates they account for 90 percent of the population.

"Amchide is Boko Haram's local fiefdom," said Lieutenant-Colonel Thierry Foumane, the regional commander of Cameroon's Rapid Intervention Battalion (BIR), an elite army unit recently deployed to the remote area.

Cameroon, like other countries in the region, has greatly stepped up its operations against Boko Haram since the April kidnapping of more than 200 schoolgirls in northern Nigeria sparked international outrage.

The Islamist group has been terrorising Nigeria with almost daily attacks in recent weeks as part of a bloody five-year insurgency that has already claimed thousands of lives.

Militants often use Cameroon as a staging post to launch attacks and kidnappings in Nigeria as the border is extremely porous, with no buffer-zone clearly separating the two countries.

When leaving Cameroon, the boundary is marked by a simple iron bar followed by a sign announcing the Federal Republic of Nigeria.

A Cameroonian army officer pointed out a house that literally straddled the border. "Look at this house. The living room is in Cameroon, but the other rooms are in Nigeria," he said.

Further complicating the operation is that Boko Haram has infiltrated families: "In the same family you can find honest people but also Boko Haram members," Foumane said.

"Our greatest challenge is to win people's confidence so they will report the bad apples. But how can you expect a parent to hand over his son, or a youth to turn in his brother?"

The situation also makes it easy for the Islamists to work their way into Amchide, a police officer with long experience in the area said on condition of anonymity.

He described them as ruthless, with beatings and blackmail in their arsenal.

For them, "anything goes in Amchide", he said. "They have offered money to youths to rally them to their cause, and they have coerced the more fearful ones into joining them."

- Alliances with businesses -

He said the Boko Haram infiltrators have also developed alliances with businesses and "forced others to finance their activities".

Since Boko Haram began its deadly campaign, the Cameroonian city has become a hub of trafficking -- in weapons, cars or manufactured goods -- which the Nigerian extremists use to finance their recruitment.

Amchide has also been a staging ground for attacks on Banki.

"We have faced Boko Haram actions since 2009," Foumane said, adding that the menace finally prompted the army to deploy the elite BIR to the border area.

He described Amchide as a forward base under the control of the Cameroonian army, while many other military bases have been set up in the far northern region.

The police officer said the army was "cleaning up" Amchide. "Several dozen" people, either suspected Islamists or businessmen suspected of collaborating with them, have been arrested, he said.

"The wave of arrests continues," he added.

"The BIR guys are working day and night. They are purging the city. There were too many bad people," a customs officer said, also requesting anonymity. "The Islamists are dispersing little by little to the villages."

Some residents hailed the military operation.

"Before there were a lot of bad people. Now they are hunting them," an elderly man said.

But the soldiers deployed here, facing frequent harassment from the insurgents, know their mission is far from over. "A Boko Haram member is never far away," a top army officer said.