Nigeria-based Boko Haram jihadists are behind horrendous violence in neighbouring Cameroon, where they have kidnapped more than 1,000 children and used some youngsters as human shields, a top UN official said.

"The system they use is just inhuman," Najat Rochdi, UN humanitarian coordinator for Cameroon, told AFP in an interview in Geneva this week.

The north of the west African country borders the area in northeastern Nigeria where a violent Boko Haram insurrection has killed more than 15,000 people since 2009.

Starting last July, the jihadists began launching cross-border attacks, initially just hit-and-run strikes to grab food, Rochdi said.

But the attacks soon escalated with the militants burning villages and killing people, and, by the end of the year, kidnapping children.

"The information I have is around 1,500" have been taken since then, she said, adding that they were mainly used as servants, to help carry tents and fetch water.

- Children on the frontline -

At the height of the attacks in northern Cameroon in February, Boko Haram deployed children on the frontline, Rochdi said.

"To my knowledge, the children were used as human shields ... (and) were aged between eight and 12," she said.

In those attacks, which are no longer taking place, the children were backed by locally recruited youths, with hundreds of heavily-armed militants taking up the rear, she added.

"The worst was the children... Obviously this created a horrible situation," she said, adding that many Cameroonian soldiers had been deeply traumatised by having to face children on the battlefield.

Rochdi said it remained unclear if the children were from Cameroon, Nigeria or elsewhere.

A four-nation fightback by Nigeria, Niger, Chad and Cameroon halted the attacks for a while, but Rochdi said the jihadists had resumed weekly cross-border hit-and-run style attacks.

"We don't feel in Cameroon that it is over at all," she said.

While hailing new Nigerian President Muhammadu Buhari's commitment to eradicating the group, she voiced concern that the onslaught might push more jihadists into Cameroon.

That would be disastrous for the country, which is already struggling to accommodate hundreds of thousands of refugees from Nigeria and conflict-ravaged Central African Republic, in addition to nearly 100,000 Cameroonians displaced since the Boko Haram attacks began, she said.

Some 240,000 Central Africans have taken refuge in Cameroon since the conflict in their country escalated in 2013, joining nearly 100,000 already there, and around 70,000 Nigerians have flooded in since last year.

This has put huge pressure on resources in a country already hit by recurring natural disasters like floods and droughts, which have left more than a million Cameroonians wondering where their next meal will come from.

More than 200,000 children in the country were malnourished, Rochdi said.

She warned that the UN had received just 31 percent of the $264 million it had appealed for to help Cameroon this year, resulting in cuts to food rations and to education programmes.

She appealed to donors to step up their efforts, warning the multiple humanitarian crises in Cameroon were creating "fertile ground for recruitment for Boko Haram."

Rochdi cautioned that without more help, Cameroon risked "becoming a real threat to the stability of the whole region."