Nigeria sent back to Benin on Wednesday 120 children who were smuggled into the country to work as slave labour, breaking stones at quarries, Benin government officials said.
The children were aged between four and 13 and had been kidnapped from their parent, they told IRIN.
This was the second batch of child workers to be repatriated from Nigeria to Benin since the two countries established joint border patrols in August to crack down on smuggling and banditry.
A first group of 116, aged between 10 and 12, was repatriated on 26 September.
Six Benin nationals and three Nigerians have been arrested on charges of child trafficking, which is rampant in many countries of West Africa. The Nigerian authorities say up to 6,000 enslaved children could be repatriated to Benin in the coming months.
Police sources said the children were found living in crude camps in the bush in terrible conditions in Ogun, Osun and Oyo states in southwestern Nigeria. They were malnourished and slept in the open and at least 13 of their companions had died over the past three months.
In early August, Nigerian President Olusegun Obasanjo closed the border with Benin in protest at the Benin government's lack of cooperation in tackling problems such as people trafficking and the smuggling of cars stolen in Nigeria into Benin where they were resold on a flourishing second hand market.
The border was reopened a week later following a summit between Obasanjo and Benin's president, Mathieu Kerekou, at which the Nigerian leader extracted pledges of stronger cooperation.
The police forces of the two countries subsequently set up joint border patrols and Benin took steps to detain and extradite known criminals wanted in Nigeria.