Fighters from Nigeria's Boko Haram Islamist sect attacked a prison over the border in Niger overnight, but were repelled, a military source there said on Monday.

The assault was the third on the Niger border town of Diffa in four days and comes as lawmakers in Niger are due to vote on whether to join a regional offensive against the militants.

"The attack was repelled. Clean-up operations have been under way since this morning," the source, one of around 3,000 Niger soldiers sent to the volatile border region, told Reuters.

Boko Haram, which is seeking to carve out an Islamic emirate in northeastern Nigeria, has stepped up its campaign of violence over the past year.

It has expanded its operations zone into neighbouring Cameroon and attacks near Lake Chad - a crossroads between Nigeria, Chad, and Niger - have sent tens of thousands of refugees fleeing across the borders.

On Saturday, the governments of Cameroon, Chad, Niger, Nigeria and Benin agreed to establish an 8,700 strong regional force to take on Boko Haram.