LAGOS (Reuters) - Nigeria's army said it had launched an offensive to push Boko Haram militants out of a remote northeast town near the border with Niger, as it fought to end a seven-year insurgency destabilizing the region.

The military said it had killed several militants in Mallam Fatori near Lake Chad but Boko Haram had called in reinforcements. "The operation is continuing," said army spokesman Sani Usman.

There was no immediate comment from Boko Haram, which does not speak to the media and only releases video statements.

But Islamic State, to whom Boko Haram pledged loyalty last year, said militants had attacked an army convoy in the area on Monday and killed 40 troops, in a statement on its affiliated news agency Amaq, picked up by SITE which monitors jihadist announcements.

There was no independent verification of either the army or the Islamic State account.

Boko Haram controlled a swathe of land in northeast Nigeria around the size of Belgium at the start of last year, but has been pushed out of most of that territory by the Nigerian army, aided by troops from neighboring Cameroon, Niger and Chad.

The militants have nevertheless continued to carry out suicide bombings in northeast Nigeria and neighboring countries, in their struggle to set up an Islamist state.

The Boko Haram insurgency has killed thousands and displaced more than 2 million people in Nigeria.

A U.N. official said on Friday more than 6 million people are "severely food insecure" and 568,000 children acutely malnourished in the nations around Lake Chad.

(Reporting by Ulf Laessing and Sami Aboudi in Dubai)