NIAMEY - Rescuers have found the bodies of 92 migrants, most of them women and children, strewn across the Sahara desert in northern Niger after their vehicles broke down and they died of thirst, authorities said on Thursday.

Rescue worker Almoustapha Alhacen said the bodies of 52 children, 33 women and seven men were found close to the Algerian border, some 160 km (100 miles) north of the mining town of Arlit in northern Niger.

Many of the victims, all believed to be from Niger, were in an advanced state of decomposition and had been partly devoured by animals, probably jackals, he said.

Niger's government declared three days of national mourning starting on Friday and said it would launch a crackdown on the networks ferrying migrants across the Sahara.

Northern Niger lies on a major corridor for illegal migration and people-trafficking from sub-Saharan African into north Africa and across the Mediterranean into Europe.

Most of those who make the perilous journey on ancient open-topped trucks are young African men in search of work. Rescuers said the doomed convoy was puzzling.

"It's the first time I've seen anything like it," Alhacen told Reuters by telephone from Arlit. "It is hard to understand what these women and children were doing there."

Rescuers found many small writing slates among the luggage, suggesting the children may have been students in a Qu'ranic school being taken to Algeria, perhaps to beg, Alhacen said.

Alhacen said 19 of the group had reached Algeria by foot and were repatriated to Niger by authorities there. Two survived after walking dozens of kilometres (miles) across the burning desert back to Arlit.

The bodies of 87 of the victims were buried on Wednesday in accordance with Islamic custom, he said.