An Italian prosecutor has evidence of phone calls between Libyan people smugglers and aid groups operating rescue boats, he told newspaper La Stampa, amid growing criticism of non-governmental groups saving refugees off the Libyan coast.

Carmelo Zuccaro, the chief prosecutor of the Sicilian port city of Catania, did not say he would open a criminal investigation, and he gave no details about the evidence.

"We don't know if we can use this information during a trial," he said.

He was not immediately available for comment on Sunday.

Italy has become the main route for migrants seeking to reach Europe, with 181,000 arrivals last year and some 4,600 estimated deaths at sea.

So far this year arrivals are up more than 40 percent on 2016, and as many as 1,000 have died, the International Organisation for Migration says.

Mr Zuccaro launched a fact-finding investigation into the work of NGO boats in February, and in March told Italy's parliament he was "convinced" smugglers were in direct contact with rescuers, though at that time he said he had no proof.

NGOs, including Save the Children, Proactiva Open Arms and SOS Mediterranee, have rejected the accusations, saying their only objective is to save lives.