Cameroon's military are battling cross-border raids by Nigeria's militant Islamist group Boko Haram, as the BBC's Thomas Fessy found out when he joined soldiers on a patrol.

A soldier is standing on the back of a flatbed pick-up truck leading the convoy.

His high-powered twin-barrelled gun is turned towards Nigeria. In reality, though, the weapon is aimed at what Boko Haram call their "caliphate", or Islamic state.

The border village of Amchide is mostly deserted. Only a handful of people can be seen as we drive through.

They are hastily throwing a few belongings on a cart as they prepare to leave. They probably did not have time to take anything when they fled during an attack, and came back to recover their possessions.

The dusty road is the line that the militants keep crossing on almost a daily basis now, attacking the villages and Cameroonian army positions.

"Every day, there are gunshots," a Cameroonian commander says. He explains that the situation is so tense that he would rather stay anonymous.

"They are there; they are turning, watching, trying to know what we are doing and how we can react. It's unpredictable. Boko Haram is like a ghost."

'Not our war' The strain is tangible. Cameroon's elite Rapid Intervention Battalion, commonly known by its French acronym BIR, has lost dozens of men since the beginning of the year in the fight against Boko Haram.

About 1,000 men from BIR, trained by US and Israeli forces, have been deployed along a 500-km (300- mile) stretch of porous border with Nigeria.

Boko Haram is advancing and Cameroon's military fight daily battles to keep the boundary with Nigeria - Africa's most populous state - intact.

Cameroon's military recently dispatched another 2,000 soldiers to the border region to reinforce troops.

Last month, Boko Haram attacked the military post at Amchide with a tank. A car bomb exploded a few metres away minutes before the tank stormed the gate of the Cameroonian base. The tank's charred remains are still to be seen outside the military post.

Cameroonian soldiers complain that they have been left to fight a war which started in another country on their own.

On the other side of the front line, the Nigerian army has fled.

"And the French, where are the French?" an army officer bitterly asks, referring to the French counter-terrorism force commanded from Ndjamena, the capital of Chad, only a few hundred kilometres to the north-east.

The rains have come to an end and the rivers have dried. Cameroonian soldiers know that during the dry season, more attacks are likely.

Their concern is that the militants, who have taken control of some ten towns in Nigeria, want to raise the black flag of their "caliphate" on their territory as well.

"No one is really able to say what Boko Haram wants," says Saibou Issa, specialist in Peace and Security Studies at the University of Maroua, the capital of Cameroon's Far North region.

"But they are able to play on the fact that the cross-border security co-operation is non-existent. It is only a matter of time before Boko Haram launches a terror attack in Maroua. Then it will trigger a much bigger crisis."

There are suspicion and fear on the streets of Maroua, which has a population of more than 200,000.

"I don't feel comfortable near people whom I don't really know well," says a student at the university, "I make sure that I am home before six PM." "Never go out without your ID card," says the young man next to her as they queue to register for their exams.

"Anything can happen and if you are caught in the area when it does, without documents, you will automatically become a suspect," he says.

In this impoverished region, pastoralists and traders cannot move across the border anymore.

"In our own town, we felt like prisoners” The long-term economic impact of Boko Haram's military campaign, launched in Nigeria in 2009, is grim.

About 43,000 Nigerians have poured into Cameroon so far, according to the United Nations High Commission for Refugees (UNHCR).

'You must convert' "We expect thousands more in the coming weeks," says Samuel Cameroun at Minawao refugee camp, which currently hosts 17,000 people.

They all arrive there with stories of unspeakable suffering inflicted by Boko Haram.

"When they took over my town, we felt like prisoners," says Mariamu Ali, 26. Her husband was shot dead by Boko Haram in Gwoza, a Nigerian town with a population of more than 250,000 captured by Boko Haram in August when it declared a caliphate in areas under its control.

After a year on the run, the rest of Mrs Ali's family is finally reunited here at the camp.

From the foothills of Gwoza in Borno state, they had kept fleeing southwards, from one attack after another.

"When they ordered single and widowed women to marry fighters, I knew I had to escape," says Mrs Ali, a Muslim.

When Boko Haram raids a town or a village, it rules with guns and knives. Adamou Moussa, a Christian, shows me a large scar on his right arm.

He took a bullet during one attack, he says, and in another attack his left hand was maimed.

"They said: 'We have come purposely for you, you have to be a Muslim today'," Mr Moussa explains.

To the militants who threatened to slit his throat if he did not comply, he replied: "To be a Muslim for me is not a day job. I cannot be a Muslim. I have my religion."

A Boko Haram fighter went for his neck but he raised his hand to protect himself. One of his fingers was severed.

People like Mr Moussa are lucky to have escaped Boko Haram's rule and are safe for now but the brutality they left behind in Nigeria may soon follow them as Boko Haram has already spread its war into Cameroon.