The nearly 300 Nigerian schoolgirls kidnapped last month from their boarding school in Chibok by the Islamist militant group Boko Haram are suspected to be deep in Nigeria's northeastern Sambisa Forest, officially a game reserve but long a redoubt for smugglers and criminals hiding in its dense and thorny scrub.

With the region's remote landscape and unprotected borders, many fear that at least some of the girls may have been dispersed to neighboring Chad and Cameroon, either to other militants or into the global marketplace of children destined for every manner of work, from domestic or agricultural labor to sexual exploitation, forced begging, and child soldiering.

"I abducted your girls," said the smiling Boko Haram leader, Abubakar Shekau, in a video that was circulating shortly after the raid. "I will sell them in the market, by Allah. There is a market for selling humans." (Related: "The Words 'Boko Haram' Have Become Nigeria's National Synonym for Fear.")

There is no evidence yet that Shekau has followed through on his pledge, but "it's entirely possible, and that is a sad commentary on our times right now," says Susan Bissell, chief of child protection for the United Nations Children's Fund (UNICEF). "Anything can be bought and sold in our world, even a child."

The International Labour Organization estimates that about 1.2 million children are trafficked worldwide, and the U.S. State Department says as many as 27 million men, women, and children are victims of trafficking at any given time. In many countries "borders are porous and law enforcement is weak," says Bissell; in addition, some one billion children are currently living in countries affected by conflict, another driver in the trafficking phenomenon. (Read "The Price of Precious" in National Geographic magazine.)

Nigeria's Trafficked Children

Countries can be a source, transit, or destination country for trafficking. There are also countries—like Nigeria with its vast natural resources, wealthy classes, and travel infrastructure—that can be all three at once, says Benjamin N. Lawrance, an expert on trafficking in Africa who holds the Conable Chair in International Studies at the Rochester Institute of Technology in New York.

"Nigeria is an important location for trafficked children coming from many countries, primarily for domestic work, for mining, for laboring in cacao fields or other agricultural industry, and also for prostitution," he says, adding that prostitution is a much smaller part of the child-labor economy. Neighboring Cameroon and Chad, he says, are also source, transit, and destination countries.

The girls taken by Boko Haram, Lawrance says, could represent an "intersection of trafficking, forced marriage, and child soldiering." Much like the kidnapping of children by the Lord's Resistance Army in Uganda and other African countries, the kidnapped schoolgirls could technically fall under the definition of child soldiers if they are living with soldiers and pressed into cooking, cleaning, or portering for them or are forced to comply with the sexual demands of the militants.

The State Department rates countries using a three-tier system to measure trafficking within a country's borders. Nigeria ranks as a Tier 2 country in the most recent, 2013 report—meaning that its leadership is making efforts to fight trafficking but doesn't fully comply with the minimum standards of the Victims of Trafficking and Violence Protection Act.

As for the kidnapped girls, Rachel Yousey, the State Department's senior coordinator for trafficking in persons, says, "All we can say at this time is that they are definitely victims of kidnapping." But she adds: "What we do know about Boko Haram, in general, is that Boko Haram does forcibly recruit or abduct children for various purposes—for intelligence gathering, for spying, for transporting, portering, for burning down structures like schools and churches, for providing information before subsequent attacks. In those types of roles you would definitely classify children [who] are doing those things as child soldiers as well as trafficking victims."

View Images A child bride cowers at her husband's feet in Rajasthan, India. PHOTOGRAPH BY JODI COBB, NATIONAL GEOGRAPHIC CREATIVE

Sold for Labor or Sex

Children trafficked for labor purposes often do not get the attention that victims of sex trafficking do. Of the 4,746 trafficking convictions the State Department tracked in 2012, only 518 were for labor trafficking, despite its prevalence. "Sex trafficking crimes, whether adults or children—people often have a much more visceral reaction to and take much more seriously than labor trafficking. But that's not to say that labor trafficking victims experience any less abuse or hardship or suffering in their situations," says Yousey.

Martina Vandenberg, president and founder of the Human Trafficking Pro Bono Legal Center in Washington, D.C., says that while "statistics are generally very bad in the trafficking field," labor trafficking can be more common than sex trafficking in many countries, adding that "trafficking does not require the crossing of borders; it can be fully internal within a country." She points to a recent documentary about eastern Ghana, for example, that has highlighted the plight of up to 10,000 boys lured or sold into an fishing industry where they work 14 hours a day and are denied medical care and education.

View Images Child soldiers, recruited by a warlord named Cobra Matata, patrol the Bavi gold mine outside of Gety, Orientale Province, Democratic Republic of the Congo. PHOTOGRAPH BY MARCUS BLEASDALE, VII

Vulnerable Populations

Poverty, war, and poor law enforcement aren't the only factors leading to the exploitation of children. "Geography can play a significant role in trafficking," says Yousey. Natural resources such as mines that require labor and are far from the watchful eyes of authorities, remote areas where traffickers can operate or hide, and porous borders can all contribute to the problem.

So can natural disasters, especially when the international relief teams have already left, says Yousey, but families' lives remain upended. Once the safety net is gone, she says, "that makes people more vulnerable to promises made to themselves or their children" to migrate for work and potentially fall into exploitative situations.

"We have concerns that natural disasters in the medium and longer term can make people very vulnerable to trafficking," and for that reason, for example, "we are looking at the Philippines typhoon situation very closely," says Yousey.

Other efforts are also under way. Worldwide, some 230 million children under the age of five do not have birth certificates, a key document for tracking vulnerable populations and—as UNICEF's Bissell calls it—"a passport to protection." UNICEF has launched a program to fix that problem, and is aiming for universal registration by 2035.