Tunisia expelled more than 100 African migrants to its border with Algeria in mid-September, in a move that may signal a "dangerous shift" in policy, Human Rights Watch said on Tuesday.
Those expelled, who included children and possibly asylum-seekers, had been intercepted at sea and brought ashore by the coastguard, HRW said.
Previously they would have been released inside Tunisia but instead they were taken to the Algerian border, with some of the migrants complaining that National Guard agents had beaten them and stolen their belongings, it added.
"Only two months after the last inhumane mass expulsions of Black African migrants and asylum seekers to the desert, Tunisian security forces have again exposed people to danger by abandoning them in remote border areas, without food or water," said HRW's Tunisia director, Salsabil Chellali.
In a statement, HRW said: "These operations may signal a dangerous shift in Tunisian policy, as authorities had previously usually released intercepted migrants in Tunisia after disembarkation".
In July, Tunisia expelled hundreds of migrants from sub-Saharan Africa to its desert borders with Algeria and Libya after a Tunisian was killed in a fight with migrants in the country's second city Sfax.
The mass expulsion, which humanitarian sources said affected more than 2,000 migrants and asylum-seekers, drew condemnation from the United Nations.
Twenty-seven of those expelled died in the desert no-man's land on the Libyan border, with many more posted as missing, the sources said.
The latest expulsions come after a July 16 agreement by the European Union to help finance Tunisia's operations against irregular migration puts its policies in the spotlight.
"The African Union and governments of the people affected should publicly condemn Tunisia's abuse of fellow Africans, and the European Union should halt all funding to authorities responsible for abuse," HRW said.