ROME (Reuters) - An Italian prosecutor said on Wednesday he is investigating some members of humanitarian organizations rescuing migrants in the Mediterranean Sea on suspicion they may have cooperated with people smugglers.

Ambrogio Cartosio, chief prosecutor of the western Sicilian city of Trapani, told a parliamentary committee in Rome that the organizations themselves were not a target of the inquiry.

Suspicions arose because some rescue crew seemed to know in advance where to find the flimsy boats crowded with migrants after smugglers sent them off from north African ports, he said.

Migrant arrivals to Italy by sea are up almost 40 percent this year over the same period in 2016, and more than a half-million people have come in the past three years. Some Italian politicians have begun saying the non-governmental organizations (NGOs) run a “taxi” service bringing migrants to Italy.

“We understand that NGOs made some rescues at sea without informing the Coast Guard,” Cartosio said. Italy’s Coast Guard is in charge of coordinating all rescues in international waters off the shores of Libya.

The prosecutor gave no further details. By law, information regarding ongoing probes must be kept secret. Aid groups have strongly denied any ties to human traffickers.

NGO boats operating off the coast of Libya have come in for criticism in Italy in recent months after the prosecutor in Catania, on Sicily’s east coast, opened a fact-finding probe into possible ties between NGOs and Libya-based traffickers.

Catania prosecutor Carmelo Zuccaro has not opened a criminal probe because he says he has no evidence of actual collusion between human traffickers and rescue boat crews, although he does maintain the two sides have been in telephone contact.

But political parties including 5-Star Movement - Italy’s most popular according to polls - have seized on Zuccaro’s comments as proof of an NGO “taxi” service for migrants.

Cartosio said he had no evidence of phone contacts between NGOs and smugglers or of payments by traffickers to NGOs, another accusation Zuccaro is looking into.

“I agree 100 percent with the work of the NGOs, even if their work saves only one human life,” Cartosio said.

Separately, Italian police in the southern city of Bari said on Wednesday they arrested five people who allegedly organized passage to other European countries for African migrants rescued in the Mediterranean. Police are still hunting for 10 others.