Geneva - The UN voiced alarm on Wednesday at the many Nigerian refugees returning from Cameroon to Banki, a border town clouded by Boko Haram violence and already struggling with overstretched supplies and services."I am extremely worried that large numbers of Nigerian refugees hosted in Cameroon are again returning to northeast Nigeria - into a situation dangerously unprepared to receive them," UN refugee chief Filippo Grandi said in a statement.The Nigerian military recaptured Banki in September 2015 after fierce battles with the Islamist militants, whose insurgency has killed at least 20 000 in Nigeria alone since 2009.On June 1, UNHCR had said that more than 12 000 refugees had returned to the area in May alone, complaining about difficult conditions in the Minawao refugee camp on the Cameroonian side of the border, and insisting they needed to get home to harvest crops.They joined some 45 000 people who have fled into the town from elsewhere in Nigeria, adding to the problems caused by a lack of shelter, severe overcrowding, inadequate water supplies and sanitation facilities and insufficient aid.And in the past three weeks, another 889 refugees, most of them children, have returned to Banki, the agency said on Wednesday."It is my firm view that returns are not sustainable at this time," Grandi said, pointing out that the returning refugees had "put a strain on the few existing services." And the situation could become even more difficult in Banki once the rains begin, bringing with them an elevated risk of the spread of diseases."A new emergency, just as the rainy season is starting, has to be avoided at all costs," Grandi said.He said UNHCR was working hard to step up an information campaign in the Minawao camp "to ensure that refugees are given accurate information on the conditions in Banki before they make the choice to return." UNHCR said it was also working to help improve the situation in the town and ensure that returnees are quickly screened and receive food and shelter and other basic support.