Italy is ramping up further pressure on EU states to share the migration burden, this time by threatening to not allow ships from the bloc's Mediterranean anti-trafficking mission to transport migrants to Italian ports.

Foreign Minister Enzo Moavero fired the latest salvo in a letter three days ago informing EU foreign policy chief Federica Mogherini that Italy wanted a revision of the EU's Sophia mission in the Mediterranean.

Read more: Libya takes over from Italy on rescuing shipwrecked migrants

In the letter, which Germany's Die Welt obtained, Moavero demanded that other member states take in migrants from the anti-trafficking rescue mission. Under the terms of the naval mission, migrants automatically disembark at Italian ports.

The issue is now being discussed in EU political and security circles.

The new populist coalition government of the League and 5 Star Movement has closed its ports to humanitarian rescue ships, saying it wants a fair distribution of migrants across the EU. 

It also wants the Libyan coastguard to take the lead in intercepting migrants and bring them back to Libya. Part of Operation Sophia involves training and equipping the Libyan coast guard.

Read more: Libyan coast guard abandoned migrants to die in Mediterranean, says rescue charity

Italy playing hardball 

The threat to close Italian ports to the EU naval mission comes as Prime Minister Giuseppe Conte this week asked the European Commission to establish a "crisis cabinet" that would oversee the distribution of migrants rescued at sea. It would also help identify other ports where migrants could disembark.

The proposed body should formalize the practice of sharing rescued migrants across the EU, building on an ad hoc deal over the weekend, Conte told the Italian newspaper Il Fatto Quotidiano on Thursday.

Read more: EU should create crisis unit for migrant arrivals - Italian PM 

About 450 migrants picked up by an Italian police ship and a British ship belonging to the EU's border agency Frontex were allowed to dock in Italy on Monday.

Italy blocked the ships for two days until Conte convinced France, Malta, Germany, Spain, Portugal and Ireland to take in most of them instead of forcing Italy to process their asylum claims alone.

Read more:  Germany to take 50 migrants from Frontex ships caught in Italy-Malta row

UN High Commissioner for Refugees Filippo Grandi praised the deal, but said "solutions are needed that go beyond piecemeal or 'ship-by-ship' arrangements."

Italy has taken in nearly 700,000 migrants and refugees since 2013, mostly Africans who crossed the Mediterranean on boats from war-torn Libya.

However, migrant arrivals by sea have plummeted since last summer, in part due to the EU's Operation Sophia strengthening its external border through cooperation with Libya's coastguard to intercept migrants. 

Italy's hardline interior minister and head of the League, Matteo Salvini, wants Libya to be declared a "safe country" where migrants intercepted in international water can be returned.

In line with international law, EU policy is that migrants rescued by its naval mission in international waters must be brought to a safe port.

Italy is pressing EU states to set up migrant processing centers as agreed at a leaders' summit in June. The establishment of the asylum processing centers would be voluntary.