"We are deeply concerned about the expulsion of migrants, refugees, and asylum-seekers from Tunisia to the borders with Libya and also Algeria," said Farhan Haq, deputy spokesman for UN Secretary General Antonio Guterres.
"We reiterate the call made by the UN Refugee Agency and the International Organisation for Migration last week for an immediate end to these expulsions and the urgent relocation of those stranded along the border to safe locations."
Mr Haq said hundreds of people – including pregnant women and children – are stranded in remote border areas with no food or water. At least 11 people have already died on the border with Libya.
"All migrants, refugees and asylum seekers must be protected and treated with dignity, in full respect for their human rights regardless of their status and in accordance with international human rights and refugee law," he said.
Tunisia, a major route for migration to Europe, has come under renewed scrutiny for its treatment of migrants after hundreds of people crossed the border into Libya last week, abandoned with no provisions.
Several people, including women and children, were found dead in the buffer zone between Tunisia, Libya and Algeria., where hundreds of people were left after Tunis launched a deportation operation in early July.
The government began the deportations after clashes between locals and migrants in the city of Sfax left one Tunisian dead and renewed racial tensions and anti-migrant rhetoric.
Last week, the UN's refugee agency called for the urgent rescue of migrants on the Libyan border, while Libyan authorities posted videos of people begging for water.
Tunis responded by denying the severity of the crisis, saying the issue was being manipulated for "political motives".
According to humanitarian organisations in Libya contacted by AFP, at least 17 people have died in the past three weeks.
In a joint statement last week, UN agencies referred to the "unfolding tragedy" of migrants, refugees and asylum seekers in Tunisia's border regions.
Migrants stranded on the border with Libya told The National of having witnessed friends being beaten to death by border guards.
“Please, we need help. We do not have food or water and our phones' batteries are dying … we just want to leave here,” Kalvin from Nigeria said via WhatsApp.