Farther north, Boko Haram employs recruits from Chad to enforce its control in northeastern Nigerian towns and cities.
In Niger, the government has declared a "humanitarian crisis" and appealed for international aid to help tens of thousands of Nigerian refugees driven from their homes by the insurgency.
"We are concerned about the increasing regionalization of Boko Haram," said Comfort Ero, Africa director for the International Crisis Group.
On Sunday, Cameroon's army announced it had broken up a Boko Haram training camp in the Mayo-Danay district in the country's Far North region. The army was looking for other hideouts in the area, said Jean-Pierre Mbida, a soldier with the Rapid Intervention Battalion tasked with fighting the insurgents.
"We will continue monitoring the area in the hope of uncovering any other Boko Haram hideouts and training grounds," he added.
Ero cautioned that cooperation between the neighboring countries is weak. "None of the sides is willing to share information with the other," Ero said. "There's always been a lack of confidence in terms of shared regional security."
She said there is also distrust of the capabilities of Nigeria's once-proud military, which has been battered by Boko Haram. A court-martial this week sentenced 54 soldiers to death by firing squad for refusing to fight the extremists.
Chad responded this week by opening a regional "counter-terrorism cell" against Boko Haram in N'Djamena, Chad's capital 40 miles (60 kilometers) from the Nigerian border, said an adviser to French Defense Minister Jean-Yves Le Drian.
Boko Haram's threat to neighboring countries was highlighted on Wednesday, when some 5,000 insurgents launched simultaneous attacks on border towns in Cameroon, using a roadside improvised explosive device and attacked the main border barracks, Cameroon's Ministry of Defense said.
Cameroonian troops repelled the attacks and killed 116 militants, while losing a sergeant and a lieutenant, it said, adding that Boko Haram must have suffered additional casualties on the Nigerian side caused by Cameroonian artillery fire.
Fighters from Chad, Niger and Cameroon long have been identified among Boko Haram fighters in Nigeria. But residents fleeing Boko Haram now report that Chadian recruits are enforcing Boko Haram's rule in northeast Nigerian border towns in Borno state. People who escaped from Gajigana village, which was attacked a week ago, said fighters they called "Chadian mercenaries" have taken charge of most communities, even sitting in courts to adjudicate local disputes.
"They monitor every movement, all the things we do, the kind of people you meet with," said Kalli Abdullahi, who escaped to Maiduguri this week. If residents break the strict Shariah law "they will get you and kill you so as to instill fear in people," he said.
Nigerian government officials confirm that Boko Haram controls 12 of 27 local government areas in Borno state, as well as some in Adamawa and Yobe states. And they long have had camps in Chad, Cameroon and Niger, say experts.
The area where the four countries' borders meet is generally poor and long has been ignored by governments. Desertification has intensified tensions. High unemployment means there are groups of disgruntled youths who are an easy target for Boko Haram recruitment. Across borders, people often belong to the same tribe and speak the same local languages. Boko Haram offers signing bonuses and monthly pay to those who join, say residents.
Boko Haram leader Abubakar Shekau long has expressed his international ambitions, saying his group is fighting to make "the entire world" an Islamic state.
Analyst Ely Karmon wrote in a paper for the Terrorism Research Initiative that Boko Haram is "an immediate and infectious regional threat."
Associated Press writers Jamey Keaten in Paris and Divine Ntaryike in Douala, Cameroon, contributed to this report. Faul reported from Cambridge, England.