Hundreds of troops are trying to protect Cameroon from cross-border attacks by Islamist terrorist group Boko Haram. But a long and porous border with Nigeria is making the task almost impossible.

Fotokol, Cameroon (dpa) - A Cameroonian soldier stands behind a chest-high wall of sandbags, his machine gun pointing in the direction of Nigeria.

He's concentrating hard with his trigger finger at the ready.

The soldiers belong to an anti-terrorism unit stationed in Fotokol, a small town in Cameroon's Far North Region that borders Nigeria's Borno State, a stronghold of Islamist militants Boko Haram.

The two countries are only separated by a small bridge - at one end lies Gambaru, a Nigerian town currently under the control of Boko Haram.

"We have to secure this bridge and stop the terrorists from crossing over into Cameroon," says Major Nlate Ebale, who heads the special unit.

To stop Boko Haram from getting into Cameroon, President Paul Biya deployed roughly 2,000 troops and members of special units to the Far North last May.

Boko Haram, with its long-running insurgency in Nigeria, is reportedly trying to establish a base in Cameroon, where the insurgents can store weapons and hide from Nigeria's military. The group's fighters infiltrate into Cameroon to steal supplies, and kidnap children to use as soldiers or sex slaves.

Even with the help of special units trained in counter-terrorism, Cameroon has been struggling to secure its long and porous border with Nigeria, which runs over 2,000 kilometres from Lake Chad, along Nigeria's Borno and Adamawa States - two of the regions worst hit by Boko Haram - all the way to the Atlantic Ocean.

Earlier this month, Cameroon's military received back-up from about 2,000 Chadian soldiers, after the African Union agreed on January 31 to send 7,500 troops to the region to help Nigeria fight Boko Haram.

Key border points are now protected by tanks as well as a battalion of special air and and reconnaissance troops. Dressed in bulletproof vests and helmets, the soldiers patrol the border or take up positions in trenches, their weapons always at the ready.

But because Cameroon's north is highly populated - almost 6 million of the nation's 21 million people live here - it remains difficult to monitor movement.

"It is not easy fighting these people. We don't sleep at all. You can never tell when they will strike," says Captain Ibrahim Njankouo, assistant commander of Cameroon's Rapid Intervention Battalion (BIR).

He points northwards, where a truck with Chadian soldiers crosses the bridge in the direction of Gambaru, to reinforce troops that have been locked in heavy clashes with Boko Haram for days.

When an insurgent tries to cross the river into Fotokol a little later, a Cameroonian sniper opens fire. The Boko Haram fighter keels over, dead.

The soldier says he saw five insurgents coming out of a house carrying weapons, pointing with his binoculars towards a building across the river. When the first man tried to cross the bridge, he shot him.

The Defence Ministry says the army has killed at least 2,000 Boko Haram fighters in the past two years. Hundreds of others have been jailed. The army has lost about 80 soldiers, according to the ministry's spokesman Colonel Didier Badjeck.

Boko Haram has in turn killed hundreds of Cameroonians in border towns and villages in the Far North Region, according to Badjeck.

"We register at least one or two Boko Haram attacks in different localities a day," special operations commander Colonel Joseph Nouma tells dpa.

Most villages lie deserted.

Thousands of civilians have moved further south since the insurgents started to launch attacks in the Far North. The few who have remained in Fotokol - mostly the elderly - speak of frightening attacks.

"We were praying in the mosque, and all of a sudden, we heard them shouting Allahu akbar, God is great ... They slaughtered 37 people [in the mosque]," says resident Modou Boukar, recalling a recent attack in Fotokol during which more than 150 local people were killed.

Boko Haram, which seeks to establish a state with its very strict interpretation of Islamic law, has expanded its reign of terror to two of Nigeria's other neighbours this year - Niger and Chad.

More than 13,000 people are estimated to have been killed by the insurgents since 2009. The rebels now control towns in north-eastern Nigeria spread over an area the size of Belgium.

In January, Nigeria postponed its general election by six weeks - to March 28 and April 11 - citing security concerns about the Islamist insurgency.