The European Border Protection Agency (Frontex) said on Thursday it would investigate reports that Bulgarian police had caged migrants, after the UN also expressed its “concern”, AFP reported.

An investigation by several European media outlets claims that the migrants were imprisoned in a makeshift cage on Bulgaria’s border with Turkey “under the supervision” of Frontex agents.

“Frontex takes seriously any reports of alleged violations of fundamental rights,” the EU agency in Warsaw told AFP.

“The information you provided to us has been forwarded to Frontex’s Fundamental Rights Office, which will look into these allegations,” Frontex added in its response.

The allegations are based on an investigation published Thursday by Le Monde newspaper in partnership with Lighthouse Reports and other European media outlets.

Frontex told AFP that since last year, its human rights office has recorded ten “serious incidents” that allegedly took place on the Bulgarian-Turkish border.

The UN refugee agency, UNHCR, in turn told AFP that its representatives had not seen the images in question and could not comment on them, but that they were “deeply concerned by the allegations”.

“We call on the states involved to investigate all allegations of violations and abuses,” said the UN High Commissioner for Refugees.