GENEVA (Reuters) – Libyan security forces used “unnecessary and disproportionate” force to detain African migrants, shooting dead some of those trying to escape, the U.N. human rights office said on Tuesday as it demanded an inquiry into the violence.

Hundreds of migrants and refugees have waited outside a United Nations centre in Tripoli in recent days to seek help in escaping Libya after what aid groups called a violent crackdown in which thousands were arrested and several shot.

Migrants and asylum seekers, some of whose claims are pending, have been targeted by heavy-handed operations by Libyan security forces, U.N. human rights spokesperson Marta Hurtado told a U.N. briefing in Geneva.

“These have resulted in killings and serious injuries, a rise in detentions in appalling conditions, as well as expulsions of individuals to countries in sub-Saharan Africa without due process,” Hurtado said.

Libya’s Government of National Unity has said it is “dealing with a complex issue in the illegal migration file, as it represents a human tragedy in addition to the social, political and legal consequences locally and internationally”.

Libya has become a major transit point for migrants seeking to reach Europe in search of a better life. Some have been returned there by the Libyan Coast Guard after setting out in rickety boats.

Ministry of Interior officials first raided an informal settlement of hundreds of migrants and asylum seekers in Gergaresh west of Tripoli on Oct 1, handcuffing, arresting, and shooting or beating those who resisted, Hurtado said.

Some 500 migrants managed to escape from the Gheriyan detention center in Tripoli on Oct. 6 and “were chased by guards who opened fire using live ammunition,” killing at least four and wounding others, she said.

Two days later, another mass escape took place from the al-Mabani center, with migrants chased by security officers who shot and killed an unknown number, she said. The head of the U.N. migration agency IOM in Libya said at least six people had been killed.

“We call on the authorities to establish prompt, thorough, impartial and independent investigations into the claims of unnecessary and disproportionate use of force including the allegations of killings by the security forces and affiliated armed groups, with a view to holding those responsible accountable,” Hurtado said.

(Reporting by Stephanie Nebehay; editing by Angus McDowall and Giles Elgood)