Cameroon was on Wednesday accused of forcibly returning some 100,000 Nigerian refugees in breach of international agreements, putting them in danger from Boko Haram Islamists.
Human Rights Watch said Nigerians who had sought refuge across the border because of the Islamist violence had been abused, attacked and even sexually exploited by soldiers.
Conditions in the only official camp for Nigerians were poor, free movement was restricted and refugees were denied proper contact with UN officials, it added.
"Since early 2015, the Cameroonian authorities have summarily deported at least 100,000 Nigerians living in remote border areas back to war, displacement and destitution in Nigeria's Borno state," the rights monitor said in a new report.
"In carrying out these deportations, Cameroonian soldiers have frequently used extreme physical violence."
"The Cameroonian military's aim seems to be to clear Nigerians out of the country and dissuade other would-be asylum seekers from seeking Cameroon's protection," it added.
HRW called it a "flagrant breach of the principle of non-refoulement", which stops refugees from being forced back to a country where they are liable to be persecuted.
Nigerian refugees 'rounded up'
The UN refugee agency last year appealed on all governments not to return anyone to northeast Nigeria "until the security and human rights situation has improved considerably".
In April and May this year, some 13,000 Nigerian refugees were returned from the overcrowded Minawao camp in Cameroon's Far North region to the border town of Banki.
AFP visited Banki in June, where the returns had increased the population living at the camp for the homeless from 32,000 to 45,000, stretching resources to breaking point.
The UN high commissioner for refugees, Filippo Grandi, said at the time he was "extremely worried" by the returns and called them "unsustainable".
Nigerians had been "rounded up" into trucks and forcibly returned to camps that were "dangerously unprepared to receive them", he added.
HRW said unlawful deportations had continued but Nigeria was also complicit as it had sent military vehicles to help bring back its citizens from Cameroon.
Cameroon also blocked Nigerian asylum seekers at the border.
Both Nigeria and Cameroon are signatories to African and international agreements protecting the rights of refugees and preventing their forcible return.
HRW called for an immediate end to the deporting of Nigerian refugees, a probe into alleged abuses, plus better screening and registering of asylum seekers in Cameroon.
It also urged the international community to better fund UNHCR operations in areas affected by the conflict.
Boko Haram's Islamist insurgency began in 2009 and has killed at least 20,000 and forced more than 2.6 million from their homes.
The violence has largely been concentrated in northeast Nigeria but there have also been repeated attacks in Cameroon, as well as Chad and Niger.