An operation in which an Italian towboat rescued more than 100 people in the Mediterranean and returned them to Libya may have been in breach of international law, the United Nations has said.

According to the Spanish charity Proactiva Open Arms, the Asso 28, an oil rig support vessel, rescued 108 people from international waters on Monday and took them to Libya, their country of departure.

If confirmed, this would constitute a breach of international law, under which migrants rescued in international waters cannot be returned to a place where their lives are put in danger. Both the United Nations and European Union have acknowledged that Libya is not safe.

The Proactiva Open Arms claim was supported by Nicola Fratoianni, an Italian politician with Free and Equal, a small leftwing party, who was onboard the Proactiva rescue ship.

Italy’s coastguard said Asso 28’s rescue activities had taken place in Libyan waters, not international waters, and were “carried out under the coordination of the Libyan coastguard, which managed the whole operation”. The Libyan coastguard was not immediately available for comment.

A Proactiva spokeswoman, Laura Lanuza, said its members learned the rescue occurred in international waters because their boat was nearby and they could listen to radio communications between the Italian ship and the Libyan authorities.

A spokesman for the UN migration agency told Reuters it could not establish the location of the rescue. He said the agency was still investigating the case but confirmed the return of the migrants to Libya.

The UN refugee agency said on Twitter the operation “could represent a violation of international law”.

Two weeks ago Italy assured Germany that it would continue to accept migrants rescued at sea until an EU-wide plan on redistributing people across the continent was established. The pledge came after high-profile moves by Matteo Salvini, the interior minister and leader of the far-right League, to block rescue ships from docking at Italian ports.

In response to Proactiva on Tuesday, Salvini made no mention of the involvement of an Italian ship in the incident. He wrote on Twitter: “The Libyan coastguard has rescued and brought back 611 immigrants in recent hours. The NGOs protest and the traffickers lose business? Fine, we’ll continue in this direction!”

In another post, on Facebook, he wrote: “The Italian coastguard has not coordinated and participated in any of these operations, as falsely declared by a foreign NGO and a poorly informed leftist MP.”

Almost 350 people were taken back to Libya, a major point of departure for those seeking to make it to Europe, overnight on 30 July, according to the International Organization for Migration.

Under a controversial pact between Italy and Libya, the latter’s coastguard has the authority to rescue people at sea and bring them back to the country.