Moroccan security forces had used "unjustified" violence against migrants trying to break through a border fence between the North African country and a Spanish exclave, a local rights group said on Saturday.
On Friday, Morocco said a total of 18 migrants had died, and others among them and security forces were injured, during a mass attempt to storm the border to the Spanish exclave of Melilla.
However, an official at the independent Moroccan Association of Human Rights, known by the French acronym AMDH, believes the final death toll is higher than reported.
"The number of those killed is unclear yet. AMDH is collecting information, but the association confirms that the death toll is high among the migrants or the security forces," said Amin Abidar, the group's head, in the city of Nador near the border with Melilla.
AMDH had earlier reported that 27 people were killed in the mayhem.
Migrants had been left trapped on the ground for hours without medical assistance, Abidar told dpa.
Videos distributed by AMDH purportedly show dozens of migrants lying crammed on the ground surrounded by security forces.
One man is led away with blood on his head. One shot shows a uniformed man beating a man lying on the ground with a baton.
Abidar said the footage was taken by activists and their sympathizers.
He called for an "urgent and fair investigation" into the incident to identify those responsible.
