Abuja - Cameroon has broken international law by forcibly deporting at least 100 000 Nigerians who had fled there to escape Islamist Boko Haram militants, Human Rights Watch (HRW) said on Wednesday.
Cameroon's minister of communications declined to comment on the report, which said the army had been detaining the refugees and torturing some of them, before the authorities pushed them back into neighbouring northeast Nigeria.
Human Rights Watch said the expulsions reported since 2015 were one of the largest illegal forced repatriations it had seen in recent years, and were in "flagrant breach" of global and Cameroonian law.
Boko Haram militants have killed tens of thousands in their fight to carve out an Islamist state in northeast Nigeria. The violence has forced hundreds of thousands to flee, many across the border into Cameroon, Chad and Niger.
"Cameroon's army has been aggressively screening newly arriving Nigerians at the border, subjecting some to torture and other forms of abuse, and containing them in far-flung and under-serviced border villages," the report said.
"This policy of blocking asylum seekers from accessing protection has made it easier for Cameroon to deport them," it said, adding that the U.N. refugee agency (UNHCR) has been denied access.
The UNHCR said the report was broadly in line with its own understanding of the situation, though it could not confirm all the details in the paper.
"Forced returns in any context are of serious concern to us and in the case of Cameroon and Nigeria we have raised these concerns repeatedly and publicly," said a UNHCR spokesperson.
Cameroon signed a treaty in March with Nigeria and the UNHCR to ensure that all refugee returns are voluntary but the rights group said Cameroon's violations had continued.
"They humiliated us like animals and beat us like we were slaves," the report quoted one refugee as saying. His brother died from the resulting internal bleeding, he added.
Some deportees, including children, were so malnourished or sick they did not survive their return to Nigeria, said HRW.
Nigeria is struggling to cope with millions of people dependent on emergency relief for food, shelter and medicine, victims of an eight-year conflict in which at least 20 000 have been killed.
At least 10.7 million people are in need of assistance in the region, mainly in Nigeria but also in Chad, Niger and Cameroon, according to the United Nations.