Algeria must immediately stop collective expulsions of African migrants across its border with Niger, a U.N. report said, after rights groups accused Algiers of rounding up and expelling thousands of people into the desert.

Algeria has sent around 35,600 Nigerians back to Niger since 2014, according to International Organization for Migration (IOM) figures cited in the report, including more than 12,000 since the start of the year.

"These collective expulsions from Algeria to Niger are in utter violation of international law," Felipe Gonzalez Morales, U.N. special rapporteur on human rights for migrants, said in the report, a copy of which was provided to Agence France Presse (AFP) on Tuesday. "I call on the government of Algeria to abide by its international obligation and halt with immediate effect all collective expulsions of migrants to Niger."

The North African state has denied abandoning migrants in desolate border areas but said it faces an influx of sub-Saharan Africans crossing over from Mali and Niger. Niger is a major trafficking route for migrants trying to reach Europe, with the EU estimating 90 percent of West African migrants pass through the country before moving on to Algeria, Libya or elsewhere. Sub-Saharan migrants have regularly been rescued from the Niger desert, or their bodies discovered, after attempting to cross in soaring temperatures with little food or water.

The U.N. report said migrants are rounded up from their homes in Algeria often at night without having time to get dressed, or collect their belongings or money. Some of them have lived and worked in the country for years with children going to local schools, it said.