An Italian coastguard ship carrying more than 500 migrants, including dozens rescued by the US Navy off Libya last week, arrived Tuesday night at a port in Sicily, days after the new far-right interior minister banned NGO rescue ships from docking in Italy.

"Diciotti ship finally lands in Pozzallo taking 522 people to safe port," the UNHCR Italy tweeted.

"They were rescued in multiple operations, 42 of them survived drowning and they need urgent medical care and psychological support," the UN refugee agency said, adding that it was at the scene along with Italian authorities and humanitarian organisations.

A dozen very dehydrated migrants, including six children, three women and one man, had already been sent to Pozzallo and taken into care by the Italian Red Cross.

It is not known whether they were part of the group of 41 migrants rescued from a vessel in distress off Libya last Tuesday by the USNS Trenton, which transferred them to the Diciotti.

The crew of the US fast transport ship also spotted 12 bodies in the water but were unable to locate them during a search after the rescue, the US Navy said.

A nearby ship from the NGO Sea Watch offered to help provided it could dock with the migrants at an Italian port, which the Italian authorities refused.

Italy's far-right interior minister Matteo Salvini has warned that NGO migrant rescue ships would not be allowed to unload their "human cargo" in Italian ports.

The new anti-immigrant Italian government last week banned the French NGO-operated vessel Aquarius, with more than 600 migrants on board, from docking in Italy, causing uproar and a sharp spat with France.

Salvini, who took office on June 1, accuses the charities of being complicit with human smugglers operating in Libya.

An Italian court, however, Tuesday dropped due to lack of evidence a case in which two NGOs, including Sea Watch, had been accused of having links with human smugglers and facilitating illegal immigration to Italy.

"These people should know that Italy no longer wants to be any part of this business of clandestine immigration and they will have to look for other ports to go to," Salvini said at the weekend.

He said Tuesday Italy would submit a proposal on migration policy to its European partners in "two or three days".