Spanish public prosecutors said Tuesday they have opened an investigation into the deaths of at least 23 migrants during a mass attempt to cross into Spain's Melilla enclave.
The announcement came a few hours before the United Nations denounced what it called "excessive force" by authorities on the border between Morocco and Spain and demanded an investigation into the migrants' deaths.
The tragedy occurred at dawn on Friday when around 2,000 migrants, mostly from sub-Saharan Africa, tried to break through the fence from Morocco into the tiny Spanish enclave.
Moroccan authorities said some had fallen while trying to scramble over the fence, giving an initial toll of 18 dead, but later raising it to 23 after another five migrants died of their injuries.
They said 140 Moroccan police were wounded.
There were very few details about the incident, but Spanish media showed images of many people lying on the ground, some with bloodied hands and torn clothes.
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The death toll is by far the worst recorded in years of attempts by migrants to cross into Melilla, one of Spain's two North African enclaves which have the European Union's only land borders with Africa, making them a magnet for migrants desperate to escape grinding poverty and hunger.
In a statement, the Spanish prosecutors' office said the decision was made by Attorney General Dolores Delgado in order "to clarify what happened at the Melilla border", citing the "seriousness and gravity" of the incident and its impact on the individuals' fundamental and human rights.
"We saw the use of excessive force by the authorities, which needs to be investigated because it is unacceptable," UN Secretary-General Antonio Guterres's spokesman, Stephane Dujarric, told reporters in New York.
Earlier on Tuesday, the United Nations' rights office called for an independent investigation "as a first step towards establishing the circumstances of the deaths and injuries", spokeswoman Ravina Shamdasani said.
While it remained unclear how the deaths had occurred, Shamdasani said the office had received reports of "migrants beaten with batons, kicked, shoved, and attacked with stones by Moroccan officials as they tried to scale the barbed-wire fence" which is between six and 10 metres high.
The African Union has also called for an "immediate investigation," with AU Commission chief Moussa Faki Mahamat expressing "deep shock and concern at the violent and degrading treatment of African migrants attempting to cross an international border from Morocco into Spain".
In Morocco, prosecutors are moving to press charges against 65 migrants, mostly Sudanese, who took part in the storming of the border, a defence lawyer in Rabat said on Monday.