Troops from Chad and Niger pursued Boko Haram fighters across a northern Nigeria border area on Thursday, driving them out of a village they held there and causing some to flee into Niger, two senior Niger military officers said.

Niger and Chad are participating in a joint offensive along with Nigeria and Cameroon aimed at ending the Nigerian Islamist group’s six-year insurgency, which has spilled across Nigeria's borders to threaten regional stability.

A column of troops left the Nigerian town of Damasak, which was retaken from the militants earlier this month, in the morning to capture the Boko Haram-occupied village of Gasheger, the sources said.

A Reuters witness across the border in the nearby Nigerien town of Diffa, which has served as a staging area for the operations, heard explosions around midday as the sources said the coalition troops met resistance from the Islamists.

“We took back Gasheger,” one of the officers told Reuters.

During the clashes, Boko Haram fighters fired a mortar that landed in the village of Kalgueri, killing one woman, the sources said.

After being driven from Gasheger, some Boko Haram fighters fled across the Komadougou River into the town of Guesseri in Niger where soldiers from Niger’s elite American-trained anti-terrorist unit pursued them.

“More than 100 fighters were in Gasheger. They’re now in an area with a lot of trees,” the second source said.

The regional offensive launched this year comes as Nigeria, Africa’s most populous country and biggest economy, prepares to hold elections on Saturday. Nigeria’s elections commission postponed the polls to allow the operation to go forward.

The offensive has succeeded in driving Boko Haram from several towns and districts. But retreating Islamist fighters have slaughtered civilians and kidnapped hundreds of villagers, according to witnesses and residents.

•Text courtesy of Reuters. Photo shows Nigerien troops engaged in the war against Boko Haram.