MILAN (Reuters) - Italian prosecutors are investigating leading humanitarian organization Medecins Sans Frontieres (MSF), a legal source said on Saturday, over its role in rescuing migrants crossing the Mediterranean Sea.

Italian police on Wednesday seized a migrant rescue boat operated by German organization Jugend Rettet on suspicion of aiding illegal immigration.

A legal source now says that MSF is also part of the investigation over the alleged rescue of migrants carried out just off the Libyan coast when there was no immediate threat to their safety.

MSF said in a statement it had not been notified of the inquiry and that it stood ready to cooperate with prosecutors since similar accusations first surfaced in the press months ago.

“We hope any doubts can be dispelled soon to end this trickling of accusations that poisons the atmosphere in an ever gloomier situation,” it said.

Italy is under pressure to manage new arrivals because some 600,000 migrants have reached it by sea from North Africa since 2014. Immigration has become a potent political issue ahead of elections in spring 2018.

In a press interview on Saturday, Luigi Di Maio, who is expected to lead the populist Five-Star movement in next year’s vote, called for “an immediate stop to the sea-taxi service”. The Five-Star is ahead of the ruling Democratic Party in polls.

Rome has asked eight NGOs to sign a code of conduct for the southern Mediterranean, including a demand that they carry an armed policemen on board their boats.

Only four aid groups have agreed to it.

Italy’s Interior Minister Marco Minniti urged the four NGOs that have not signed the document to do so.

“Those who do not sign cannot remain part of Italy’s rescue system, though nobody is questioning the law of the sea and international treaties,” he said in an interview with Saturday’s Il Fatto Quotidiano.

“Because migrants arrive in Italy, we must find a balance between their rights and those of the country that hosts them. We need complete trust between those who carry out the rescues and the country that opens up its ports,” he said.

MSF said in the statement that it would respect most of the code’s tenets, but could not sign it as it neglected humanitarian principles and elements which it considers essential for rescues to be effective.