Niger’s air force has bombed a convoy of trucks carrying smoked fish to Nigeria, a trade banned by local officials because it is suspected of helping to finance the Islamist militant group Boko Haram, security sources said on Wednesday.

“Earlier this week, air forces fired on merchandise vans along the river Kamadougou,” said one of the sources. The river separates northern Nigeria from southeastern Niger.

The people in the trucks fled over the border into Nigeria, the source said.

Niger officials say Boko Haram taxes fishermen or seizes their catches along the shores of Lake Chad and uses the money to help fund their bid to carve out a caliphate in northeast Nigeria. The insurgency, which has killed thousands, was partly responsible for delaying Nigeria’s election earlier this month.

A second security source confirmed the incident and said about a dozen people suspected of involvement in trading fish for Boko Haram were arrested earlier this week.

The air strikes came as the governor of Niger’s Diffa region, which borders Boko Haram strongholds in Nigeria and has seen a spate of attacks by the militants this month, officially banned the sale and transport of fish into Nigeria this week.

Fishing is one of the main economic activities in Diffa, and much of the fish caught there is shipped to Nigeria. Boko Haram also raises cash through ransom paid for hostages and the sale of stolen cattle, security sources say.

A senior Cameroonian military officer said the army there was working with Chad to freeze the group’s trade.

“What we have been trying to do is stop any kind of economic activity that Boko Haram could be benefiting from. We are trying to choke all their resources,” he said on Wednesday.

Regional armies have massed troops along Nigeria’s border before a joint operation against the Islamists.

Although the regional force will not be operational until the end of next month, the armies of Niger, Cameroon and Chad have had some success in driving the militants back into Nigeria following cross-border attacks.

Nigeria’s army has also recaptured several towns previously held by the militants, but security sources say Lake Chad and its waterways are difficult to police.

•Text courtesy of Reuters. Photo shows a Nigerian Air Force jet on an operation.