KANO, Nigeria : Nigeria has sealed a portion of its northeastern border with Cameroon to block the movement of insurgents and other criminal groups, the military said Sunday. The closure has been imposed in Adamawa state, one of three states in the northeast placed under emergency rule in May following waves of attacks by Boko Haram. The military has launched a major offensive in the area aimed at crushing the uprising, which has killed thousands since 2009. Nigeria has alleged that the extremists have set up bases in sparsely populated areas of its northeastern neighbours, including Cameroon, Chad and Niger, and flee across the border after staging attacks to avoid military pursuit. "What I did was completely seal off the borders, no going in, no going out," said Brigadier General Rogers Iben Nicholas, the top military commander in Adamawa.He said the measure had been in place since Monday and that it has already curtailed "the influx of miscreants (and) terrorist elements" into Nigeria."Other security agencies like the customs, immigration services have been told. Our soldiers and police are also there working together to ensure that nothing crosses into Nigeria," Nicholas said.Despite the state of emergency, Boko Haram has continued to carry out attacks in the northeast, with more than 300 people killed already this year.Adamawa has been less hit by violence than the other affected states, Borno and Yobe, but it is thought to provide key transport routes for the insurgents.The full 2,000-kilometre (1,200-mile) porous border stretches from Borno down to the southern Niger Delta region.Aside from curbing the flow of suspected insurgents, the several-hundred-kilometre closure in Adamawa will also affect traders and other residents.Nicholas said the military was working with traditional rulers on both sides of the border to inform people about the closure.