An official and witnesses say Niger troops have deported more than 3,000 Nigerian fishermen and refugees escaping Boko Haram.

They said Wednesday a dozen people died during a three-day trek in severe heat.

Nigeria's National Emergency Management Agency said it was alerted by Niger and sent trucks to collect the exhausted refugees at the border.

The agency's Charles Otegbabe was registering the new arrivals in Gaidam town. He said he had listed more than 3,000 people and expected hundreds more.

Two of the refugees said about a dozen people died along the way due to lack of water, including a woman and the twins she had just given birth to.

Niger said Monday it was carrying out military operations in the Lake Chad area and ordered citizens to move inland.