The non-governmental organisations -- Doctors Without Borders (MSF), Oxfam Italia, SOS Humanity, Association for Juridical Studies on Immigration and EMERGENCY -- argue the legislation breaks EU and international laws regarding sea rescues.

"The European Commission is the guardian of EU treaties and has a role to play in ensuring that EU member states respect international and EU law," a migration policy advisor with Oxfam Italia, Giulia Capitani, said.

"Every day we spend away from the search and rescue region, whether in detention or navigating towards a distant port, is putting lives at risk," said an MSF operations manager, Djoen Besselink.

The European Commission confirmed to AFP it had received the complaint. "We will look into it," said a spokesperson.

Italy passed a law in February limiting charity-run ships carrying out more than one sea rescue at a time and -- having done so -- requires them to dock at a port assigned by Italian authorities. It came into effect in March.

In practice, the NGOs say, that means rescue ships are forced to ignore other emergencies on the water, and spend days sailing to northern Italian ports even though closer ones are available.

Italy is ruled by a far-right government that has vowed to crack down on migrant boat arrivals.

The five charities called on the European Commission to put Italy's law and designations of remote ports under "immediate scrutiny".

They also appealed for their ships to be included in a "state-led and proactive" search and rescue system in the Mediterranean.

The European Union is working on a revision of its asylum and refugee policies aimed at sharing out the burden of hosting asylum-seekers across the bloc and speeding the return of denied asylum-seekers to countries of origin or transit.