"We have noticed an influx of people from the Niger Republic who cross the border illegally. They are fleeing from the famine facing them," said Hassan Suleiman Kangiwa, head of the Nigerian immigration service in Katsina State.

"We dont have statistics of the illegal immigrants but all I can say is that their number is quite substantial and we have beefed-up security at the border posts to contain the influx," he told AFP.

Niger is one of the world's poorest countries and is facing what the United Nations World Food Programme last month dubbed a "forgotten emergency" after locust swarms and five years of virtual drought destroyed crops.

As citizens of the Economic Community of West African Statespeople from Niger have a right to travel to Nigeria, but Kangiwa said that many arriving without travel documents were being detained and sent home.

"We are cognizant of the ECOWAS treaty ... and we respect that treaty and allow people with valid documents into Nigeria," he said.

"But we will not allow anybody who doesnt possess valid travel documents into the country simply because he is from an ECOWAS state," he added.

"Although these illegal immigrants are fleeing from starvation, their influx can constitute economic, security and health risks to us which will be detrimental to our survival as a nation. This is why we are sending them back."

Last month Gian Carlo Cirri, WFP country director for Niger, said: "The situation is extremely serious; it is not yet a famine but if we do not get the resources we, and our partners, require ... we will likely face a famine."

In a good year Niger is able to grow enough grain and millet to feed its 11.5 million people and export to its neighbours. But the rains have been patchy for five years, and last year locust swarms devastated many crops.