Some have been left by traffickers, but others were deported by Algerian authorities - left in the desert and told to walk to the nearest border town.

In the searing heat, a number of people have died.

One migrant, from Mali, recounted his own experience:

They took all our belongings - money, mobiles, everything. They treated us badly. They dropped us in the Sahara Desert.

And then, we were forced to walk dozens of kilometres to reach Assamaka. the border town. It was like walking through hell. Pregnant women and underage children were trudging along as the Sahara sun was beating us down. We had nothing to eat, and we heard that at least two people died in the dunes."

Giuseppe Loprete, IOM's chief of mission in Niger, has been trying to raise awareness of their plight:Giuseppe Loprete?@g_loprete

 Saturday 343 #migrants expelled by #Algeria including 176 men, 53 women, 114 minors. Over 3000 migrants with @OIM_Niger for assistance. Many departures this week for their homes by charters and bus with #EUTF. #refugees expelled by Algeria now helped @UNHCRNiger. Will this end?

2:13 PM - May 14, 2018

Mr Loprete told the BBC's Newsday that Algeria had defended its actions as necessary to prevent insecurity and terrorism, adding it did not want anybody to cross the border without knowing who they are.

However, the IOM had found a number of people with refugee papers from Algeria among those forced to make the dangerous walk to the border.