The EU has assured the UN Security Council that "no refugee or migrant intercepted at sea will be sent back against their will".
Federica Mogherini addressed the council as the 28-member EU prepares to start making decisions next week on an operation to identify, capture and destroy boats before they are used by migrant smugglers.
Concerns remain even among some council members that the migrants themselves will be harmed, sent back or not be allowed to seek better lives.
The UN's special representative for international migration says about half of the people who reach Europe qualify as refugees.
Peter Sutherland praises a planned EU quota system where countries share the refugee settlement burden.