Mayors across the south of Italy have pledged to defy a move by the new Italian government – an alliance of the far right and populists – to prevent a rescue boat with 629 people on board from docking in the Sicilian capital.

But the mayors’ defiance appears unlikely to serve any practical purpose without the direct support of the Italian coastguard.

In the first evidence of the new government’s hardline approach, the interior minister, Matteo Salvini, said on Sunday that all Italian ports were closed to the rescue boat, Aquarius.

The Maltese government rejected a request to take the boat, saying international law required that the migrants should be taken to Italian ports.

Salvini, the leader of the League, a far-right party, wrote on Facebook: “Malta takes in nobody. France pushes people back at the border, Spain defends its frontier with weapons. From today, Italy will also start to say no to human trafficking, no to the business of illegal immigration.”

Leoluca Orlando, the mayor of Palermo, said he was ready to open the city’s seaport to allow the rescued migrants to safely disembark.

“Palermo in ancient Greek meant ‘complete port’. We have always welcomed rescue boats and vessels who saved lives at sea. We will not stop now,” Orlando said. “Salvini is violating the international law. He has once again shown that we are under an extreme far-right government.’’

Other mayors in Italy’s south, including those in Naples, Messina and Reggio Calabria, also said they were ready to disobey Salvini’s order and allow Aquarius to dock and disembark in their seaports.