Vienna - EU defence ministers have pledged their support to the EU's anti-migrant smuggling mission in the Mediterranean Sea, but have failed to resolve an impasse resulting from Italy's refusal to keep taking in migrants rescued by Operation Sophia.

The populist government in Rome has taken a hard-line stance on migration, refusing to disembark people rescued at sea unless other EU member states offer to take them in.

Reviewing the mission

The issue is threatening the future of Operation Sophia, whose current mandate is set to expire at the end of the year. EU member states are in the process of reviewing the mission.

"We registered strong determination from all member states, no one excluded, to continue the operation and keep it effective," said EU foreign policy chief Federica Mogherini, following an informal meeting of EU defence ministers in Vienna.

"This brings a collective responsibility," she added.

When Operation Sophia was established in 2015, Italy agreed to take in any migrants rescued by its vessels.

Demanding a change to the rules

Search and rescue is not a core objective of the naval mission, but it is duty bound by international rules on the need to assist anyone found in distress at sea. So far, Operation Sophia has rescued more than 49,000 migrants.

Rome is now demanding a change to the rules and calling on other member states to take in migrants, threatening to close its ports to Operation Sophia's vessels otherwise.

Options being discussed include disembarking migrants at ports in different member states bordering the Mediterranean, as well as redistributing migrants among member states once they come ashore.

However, decisions on responsibility-sharing must be taken by interior ministers or EU leaders, Mogherini noted. The issue could come to dominate an informal EU summit in the Austrian city of Salzburg next month.

Italian Interior Minister Matteo Salvini has threatened to suspend his country’s contributions to Operation Sophia, if Rome's proposals on the redistribution of migrants are not adopted.

Mission until end of the year

Speaking in Vienna, however, German Defence Minister Ursula von der Leyen appealed to Italy not to hijack the mission.

"It is also a question of credibility and the reliability of the European mission. We brought it into being. It runs until the end of the year. And it must continue until the end of the year," von der Leyen said.

At the heart of the issue are the EU's so-called Dublin rules on asylum, under which migrants must register asylum claims in the EU member state where they first set foot. Efforts to reform the rules have so far failed.

However, this row must not take place at the expense of Operation Sophia, which has an important task, von der Leyen said.