Spanish public prosecutors on Tuesday said they had 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 UN denounced "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 about 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 climb over the fence, giving an initial toll of 18 dead, but later raising it to 23 after more 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.
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 that have the EU's only land borders with Africa, making them a magnet for migrants desperate to escape poverty and hunger.
The Spanish prosecutors' office said the decision was made by Attorney General Dolores Delgado "to clarify what happened at the Melilla border", because of the "seriousness and gravity" of the incident and its effects on the people's 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, said in New York.
Earlier on Tuesday, the UN's 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, Ms 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 tall.
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 storming the border, a defence lawyer in Rabat said on Monday.
