The country has been accused of expelling the migrants without food or water and forcing them to walk, sometimes at gunpoint, under a blistering sun before being left to die.
Pictures show migrants coming over the horizon by the hundreds under temperatures of up to 48C. Officials from the International Organisation for Migration have warned of a looming 'catastrophe'.In Niger, where the majority head, the lucky ones limp across a desolate 10-mile no-man's-land to the border village of Assamaka, the Associated Press said.
Others are reported to have wandered for days before a UN rescue squad can find them.
Untold numbers die, with nearly all of the more than two dozen survivors interviewed by The Associated Press telling of people in their groups who simply vanished in the Sahara.
'Women were lying dead, men… Other people got missing in the desert because they didn't know the way,' said Janet Kamara, who was pregnant at the time. 'Everybody was just on their own.'
She recalled at least two nights in the open before her group was rescued, but said she lost track of time.
Another woman in her early twenties also went into labour and lost her baby, she said.
Algeria's mass expulsions have picked up since October 2017, as the European Union renewed pressure on North African countries to head off migrants going north to Europe via the Mediterranean Sea or the barrier fences with Spain.
A European Union spokesman said the EU was aware of what Algeria was doing, but that 'sovereign countries' can expel migrants as long as they comply with international law.
Unlike Niger, Algeria takes none of the EU money intended to help with the migration crisis, although it did receive 111.3 million US dollars in aid from Europe between 2014 and 2017.
Algeria provides no figures for its involuntary expulsions, but the number of people crossing on foot to Niger has been increasing since the International Organisation for Migration started counting in May 2017 to as high as 2,888 in April 2018.
In all, according to the IOM, a total of 11,276 men, women and children survived the march.
At least another 2,500 were forced on a similar trek into neighbouring Mali, with an unknown number succumbing along the way.
The migrants AP talked to described being rounded up hundreds at a time, crammed into trucks for hours to what is known as Point Zero, then dropped in the desert and pointed towards Niger.
'There were people who couldn't take it. They sat down and we left them. They were suffering too much,' said Aliou Kande, an 18-year-old from Senegal.
Mr Kande said nearly a dozen people gave up, collapsing in the sand. His group of 1,000 wandered from 8am until 7pm, he said. He never saw the missing people again.
'They tossed us into the desert, without our telephones, without money,' he said.
The migrants' accounts are confirmed by videos collected by AP over months, which show hundreds of people stumbling away from lines of trucks and buses, spreading wider and wider through the desert.
'You're facing deportation in Algeria — there is no mercy,' he said. 'I want to expose them now… We are here, and we saw what they did. And we got proof.'
Algerian authorities refused to comment. But Algeria has in the past denied criticism that it is committing rights abuses by abandoning migrants in the desert, calling the allegations a 'malicious campaign' intended to inflame neighbouring countries.
The Sahara is a swift killer that leaves little evidence behind. The International Organisation for Migration has estimated that for every migrant known to have died crossing the Mediterranean, as many as two are lost in the desert – potentially upwards of 30,000 people since 2014.
The vast flow of migrants puts an enormous strain on all the points along the route.
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He then tries to arrange rescue for those still in the desert.
'It's a catastrophe,' he said.
Most choose to leave by IOM bus for the town of Arlit, about six hours to the south through soft sand. And then on to Agadez, the Nigerien city that has been a crossroads for African trade and migration for generations.
Ultimately, they will return to their home countries on IOM-sponsored flights.
Even as these migrants move south, they cross paths with some who are making the trip north towards Algeria and Europe.
