Rome plans to ask the European Union to modify the rules of the Sophia mission -- currently commanded by Italy -- and rotate the ports where migrants rescued at sea can disembark, with France and Spain expected to top the list.

Currently all the ships dock in Italy but Rome's new right-wing, nationalist government says it should not have to carry the burden on its own and it is time other EU states do their fair share by taking in more of the migrants.

In comments before informal talks by EU defense ministers, Mogherini called on them to show a "constructive attitude" to work to continue the mission.

"So far consensus has not been found... We can definitely not afford to leave an EU operation without clarity on the rules it has to follow," she said ahead of the meeting in Vienna.

"It would be good if member states take more responsibility," she added. "The important thing is that we manage to keep the operation going... This has been a remarkable achievement for all of the European Union."

German Defense Minister Ursula von der Leyen said Sophia's mandate was until year end -- when she expected EU leaders to solve the question of how asylum seekers coming to Europe whose claims are recognised should be distributed among member states and how those rejected should be returned home.

"That is the question that is anyhow right on top of the agenda of EU leaders... and so I expect this question to be solved in the autumn," she said.

EU leaders will meet in the Austrian city of Salzburg in September to discuss the migrant crisis. Austria currently holds the EU's rotating presidency.

Sophia was launched in June 2015 following a series of deadly shipwrecks and has since picked up thousands of migrants floundering in the Mediterranean.

According to La Stampa newspaper, Italy's idea is to rotate landings between Mediterranean ports, with a particular emphasis on France and Spain, and with Greece and Malta also sharing the load.

Italian Defence Minister Elisabetta Trenta said late Wednesday that the ball was in the EU camp.

"By accepting our proposal it (the EU) will have the opportunity to show it is a real community of values and intentions; by refusing it will deny its own fundamental principles," she said.

Italy has been turning away ships with migrants rescued at sea in a campaign to make EU countries take their share.

Last week, it threatened to stop billions of euros of EU funding over the issue, accusing Europe of turning its back as Italy grapples with seemingly endless migrant arrivals.