The Associated Press interviewed over two dozen survivors of the deportations in Niger.

Nearly all said they saw fellow migrants collapse during the walk, where temperatures reach up to 48C. They never saw the missing migrants again.

The lucky make it within a few hours to the nearest village across borders in Niger and, more recently, Mali. But many wander for days.

Algeria denies mistreating the migrants.

But their accounts are confirmed by multiple videos collected by the AP showing hundreds of people stumbling into empty desert.

"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."

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 spokesperson 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 $US111.3 million ($A149.8 million) 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 Organization for Migration started counting in May 2017, when 135 people were dropped, 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.

Australian Associated Press