Madrid will offer "total collaboration" with the Spanish and Moroccan investigations into the deaths of 23 migrants during a mass attempt to enter Spain's Melilla enclave, Pedro Sanchez said Wednesday.

The Spanish premier's remarks came a day after the United Nations denounced authorities on the border between Morocco and Spain for using "excessive force", describing it as "unacceptable".

The tragedy happened at dawn on Friday, when around 2,000 migrants, mostly from sub-Saharan Africa, tried to break through the fence from Morocco into Melilla, one of Spain's two tiny North African enclaves.

"I regret the loss of human life and express my solidarity with the families of the migrants who died," Sanchez told Cadena Ser radio, pledging his government would work with investigators to understand what happened.

Sanchez stressed that three investigations had been opened, one by Moroccan prosecutors, one by Spain's public prosecutor and a third by the Spanish rights ombudsman.

"We have to trust these institutions and I pledge the government's total collaboration with their efforts to clarify what happened," he said.

Moroccan authorities said some of the victims 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.

Few details about the incident were available, but Spanish media showed footage of people on the ground, some with bloodied hands and torn clothes.

The death toll was by far the worst recorded in years of attempts by migrants to cross into Spain's Ceuta and Melilla enclaves, which have the EU's only land borders with Africa, making them a magnet for those desperate to escape grinding poverty and hunger.

In Morocco, prosecutors are pressing charges against 65 migrants, mostly Sudanese, for trying to storm the border, a defence lawyer in Rabat said.

Twenty-three migrants died during a mass attempt to enter Spain's Melilla enclave on Friday
Twenty-three migrants died during a mass attempt to enter Spain's Melilla enclave on Friday AFP / FADEL SENNA

Spain's public prosecutor on Tuesday opened its own investigation "to clarify what happened", citing the "seriousness and gravity" of the incident.

Images of the violence provoked an unusually strong response from the United Nations, which hit out at the border authorities.

"We saw the use of excessive force by the authorities, which needs to be investigated because it is unacceptable," said UN Secretary-General Antonio Guterres's spokesman Stephane Dujarric on Tuesday.

"People who are on the move have human rights and those need to be respected and we're seeing them all too often disrespected. We've seen it on both sides of the border," he added.

Sanchez had on Friday blamed "human trafficking mafias" for what he said was "a well-organised violent assault" and defended the actions of the border police, saying the incident was "well-handled by the security forces of both Spain and Morocco".

Asked about his initial defence of the security forces' actions, Sanchez said that when he spoke, he was "not aware of the reports and images" of the victims.

And on Wednesday he reiterated concern for the border security agents, giving a figure of 40 injured Spanish police and 100 on the Moroccan side.

"I also ask that we put ourselves into the shoes of the injured police and security forces, in both Morocco and in Ceuta and Melilla, who have the right to an orderly flow of migrants and not be at the mercy of violent attacks," he said.