The African Union chief has expressed “deep shock” over the death of nearly two dozen people while trying to break through a border fence between Morocco and the Spanish enclave of Melilla and called for an immediate investigation into the matter.

Moussa Faki Mahamat, chairman of the African Union Commission said on Monday that he was shocked at the “degrading treatment of African migrants”.

“I express my deep shock and concern at the violent and degrading treatment of African migrants attempting to cross an international border from #Morocco into #Spain, with the ensuing violence leading to the deaths of at least 23 people and injuries to many more,” Mahamat said in a tweet.

Morocco’s interior ministry said in a statement that 23 people died on Friday due to what it described as a “stampede” after about 2,000 refugees and migrants tried to climb the metal border fence.

The Moroccan authorities said that 140 police officers were also wounded in the violence.

Mahamat called for “an immediate investigation into the matter and remind all countries of their obligations under international law to treat all migrants with dignity and to prioritize their safety and human rights, while refraining from the use of excessive force.”

Many asylum seekers were seen lying on the ground, some wounded, in photos that were widely shared on social media. Activists have called their treatment “cruel, inhuman and degrading”.

Friday’s incident had the highest death toll in years of such attempts to cross the frontier at Melilla, a common crossing point for refugees and migrants seeking a better life in Europe.

Melilla and Ceuta are the European Union’s only land borders with Africa and are protected by fences fortified with barbed wire and surveillance cameras.

The deadly crossing was the first since Spain and Morocco mended relations earlier this year, after a spat triggered by Rabat laying claim to the disputed region of Western Sahara.

On Sunday, the Moroccan Association for Human Rights (AMDH) published an image showing authorities from the North African country digging about 20 graves, which the organisation says were meant for the asylum seekers who died in the crossing.

Last week, several non-governmental organisations released a joint statement demanding a probe into the treatment of refugees and migrants as they attempted to cross the border. The AMDH specifically pleaded against burying the bodies without an investigation.

Meanwhile, Spanish Prime Minister Pedro Sanchez has applauded Spanish and Moroccan collaboration on the border, saying the mass migration attempt was “well resolved”. Sanchez also called the incident “an attack on the territorial integrity of our country in a violent manner”.