Italy's new populist government will refuse to let a humanitarian boat carrying more than 600 refugees and migrants dock at any of its ports and has asked the tiny Mediterranean country of Malta to open its doors to the vessel, according to media reports.

Malta said it had nothing to do with the rescue operation, opening the prospect of a diplomatic standoff between the two European Union allies.

"From now also Italy begins to say NO to the traffic of human beings, NO to the business of illegal immigration," Matteo Salvini, Italian interior minister and head of the far-right League party, wrote on Twitter on Sunday.

"My goal is to guarantee a peaceful life for these children in Africa and for our children in Italy," added Salvini, who is also a deputy prime minister after campaigning on a staunchly anti-immigrant platform, vowing to "put Italians first" and send hundreds of thousands of undocumented migrants "home".

More than 600,000 people have reached Italy by boat from Africa in the past five years. Numbers have dropped dramatically in recent months, but there has been a rise in rescues in recent days, presenting Salvini with his first test as minister.

Earlier on Sunday, the charity SOS Mediterranee on Twitter that its rescue ship, The Aquarius, had taken on board 629 refugees and migrants, including 123 unaccompanied minors, 11 other children and seven pregnant women.

The charity said the group of mainly sub-Saharan Africans were picked up in six different rescue operations off the coast of Libya and included hundreds who were plucked from the sea by Italian naval units and then transferred to The Aquarius.

"The boat is now heading north towards a secure port," SOS Mediterranee tweeted on Sunday without specifying its destination, though virtually every such boat over the past five years has ended up in Italy.

'Letter sent to Malta'

Its route north will take it past Malta, and an official said that Salvini had written to the government of the small island state asking it to let The Aquarius dock there.

The official, who asked not to be identified, said that Italy would not open its ports to the NGO's vessel, according to the Reuters news agency.

Italian media also reported the same letter.

According to the daily Corriere Della Sera, the letter describes Malta's capital, Valletta, as the "most secure port" and the boat should thus dock there. La Repubblica also published a story about the letter.

There was no comment from the Italian Ministry of the Interior or the local coastguard.

Malta said the rescue operations happened in international waters off Libya and were coordinated by Italy.

"Malta is neither the competent nor the coordinating authority in this case. Malta will observe prevailing laws," its government said in a brief statement.

Salvini, 45, has continued with his anti-refugee rhetoric since taking control of the interior ministry, asking on Friday for NATO to defend Italy, which he says is "under attack from the south".

His stance on immigration has been enshrined in the 57-page contract the League agreed to with government coalition partner Five Star Movement. Headed by Prime Minister Giuseppe Conte, the government was inaugurated last week.

The immigration section of the policy document calls for the deportation of Italy's estimated 500,000 undocumented immigrants "as a priority", building more detention centres and a review of the European Union's Dublin Regulation, which stipulates that migrants and refugees apply for asylum in the first EU country they reach.

Salvini's comments on Sunday came after another spat with Malta following its reported refusal to come to the aid of another rescue ship, Seefuchs, which was stranded with 126 refugees on board due to violent seas until it was allowed to dock in the Sicilian port of Pozzallo on Saturday.

Al-Jazeera