NIAMEY (Reuters) - Nigeria's army offensive against Islamist militants has pushed nearly 40,000 refugees over its northern border into Niger, a U.N. agency said, in a drive that is straining food supplies in the drought-prone country.
The United Nations estimated in June there were 6,000 refugees from Nigeria but the figure has soared as President Goodluck Jonathan has stepped up attacks on Boko Haram militants.
The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said in its latest monthly report that of a total 37,332 refugees, nearly 29,000 are officially Niger nationals and the rest are Nigerian.
"These figures, three times above the level the humanitarian workers were planning for, give an indication of the difficulties of developing a humanitarian response," it said.
Boko Haram is seen as the biggest risk to stability in Nigeria, Africa's most populous country and top oil producer, which shares a 1,500 kilometer border with its landlocked northern neighbor Niger along the edge of the Sahara Desert.
The United States formally designated Boko Haram and splinter group Ansaru as terrorist organizations on Wednesday.
OCHA said the refugees in Niger were spread over 20 different areas in the semi-desert southern province of Diffa and that they were living mostly with local hosts.
A spokesman for the government in Niger declined to comment. A Nigerian defense spokesman said he had no information about refugees crossing the border. Nigerian emergency service officials were not available for comment.
The rising number of refugees in Niger is expected to put further strain on food supplies after a disappointing harvest.
According to an October report by FEWS NET, a USAID-funded famine network, 1.2 million people in Niger will be in a situation of acute food insecurity from January 2014.
Thousands of refugees have also spilled into Cameroon, prompting Nigeria to reach out for help in policing their shared border.
The United Nations has called on neighboring countries to keep their borders open and has urged Niger to grant refugee status to the Nigerian nationals in the Diffa region.
(Reporting by Abdoulaye Massalatchi; Additional reporting by Tim Cocks; Writing by Emma Farge; Editing by Gareth Jones)