ROME (Reuters) - 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 Organization for Migration says.

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.

Zuccaro’s comments come two days after Italy’s anti-establishment 5-Star Movement said NGOs were providing a “taxi service” for migrants, adopting a tough anti-immigrant line similar to right-wing parties such as the Northern League and Forza Italia.

Italy is due to hold a general election before next summer, and most polls show the 5-Star Movement has a narrow lead on the ruling center-left Democratic Party.

EU border agency Frontex has also criticized the NGOs, saying they make it too easy for smugglers.

Humanitarian rescue boats wait just outside of Libyan territorial waters. Migrants are packed by smugglers onto flimsy rubber boats that struggle to make it to international waters before sinking.

“More would die if we weren’t there,” Chris Catrambone, who co-founded the MOAS NGO to rescue migrants with his wife Regina, told Reuters.

During a visit to Canada on Friday, Prime Minister Paolo Gentiloni cautioned against criminalizing NGOs, saying they “save lives and should be thanked”.