"The International Convention for the Safety of Life at Sea dictates that any ship learning of distress at sea must assist regardless of the circumstances".
The European Commission had urged member states to act with responsibility, calling on both Italy and Malta to respond.
"What does worldwide law say?"
He added: "It will be necessary to sit down and discuss how to prevent this kind of thing from happening in the future".
Given that the aid ship had no emergency, Italy made a decision to appeal to other European countries "so they don't leave Italy alone yet again in managing the migratory flows, which is a phenomenon that is all of Europe's business", Di Maio said in a Facebook post.
"Plan from MRCC (the Maritime Rescue Coordination Centre) Rome is that rescued people will later be transferred on Italian ships before heading together to" the eastern Spanish port of Valencia, rescue charity SOS Mediterranee said on Twitter.
Salvini refused to back down.
Matteo Salvini, Italy's new interior minister, prevented the passengers from disembarking in his country on Sunday.
UNHCR also pointed to "key treaties" stating that a nation which has responsibility for an area in which a search-and-rescue operation takes place is required to "exercise primary responsibility" for coordinating the migrants' safe disembarkation. Although there were no immediate emergencies on board, the spokesman said, there were many who needed medical care for chemical burns, hypothermia and dehydration.
Malta on Sunday reiterated its refusal to accept the ship, ignoring calls from Rome.
Mayors of Naples and Palermo in turn vented their anger at Salvini, as they stated that their cities would be more than happy to accept those in need of shelter despite the fact that both cities had been suffering from major migrant influx in the past five years.
In a Facebook post dated April 21 a year ago, M5S leader Luigi Di Maio called boats carrying stranded migrants to Italy a "sea taxi", claiming they were conducting services they said were charitable in exchange for money.
The Italian government, and others like it including Britain's, want hundreds of people to drown to deter others from trying to flee to Europe to escape war, poverty and climate change.
Italy demands an equal share of migrant intake to be distributed across the bloc, something Conte raised at the G7 meeting this past weekend.
European Union leaders in December had set an end-June deadline for an overhaul of rules to create a permanent mechanism to deal with migrants in the event of a new emergency.