Rome (AFP) - The United States on Monday offered its backing for a NATO naval operation off Libya in support of a controversial Italian plan to close the Western Mediterranean migrant route to Europe.

"Barack Obama said he was willing to commit NATO assets to block the traffic in human beings and the people smugglers that we refer to as modern slavers," Italian Prime Minister Matteo Renzi told reporters after meeting the US president and the leaders of Britain, France and Germany in the German city of Hanover.

The talks touched on the migrant crisis as well as instability and Islamist infiltration in Libya, a departure point for 350,000 people who have set sail to Italy since the start of 2014.

Italian defence minister Roberta Pinotti had earlier revealed that preparations for a naval blockade are already advanced, with approval expected when NATO leaders meet on July 7 in Warsaw.

US officials confirmed that Washington is fully on board.

The White House said Obama and the other leaders want to learn from efforts to shut down Europe's other major migrant conduit -- the route from Turkey to Greece -- to plan the operation.

NATO already has an operation to stop migrant boats reaching the Greek islands from Turkey.

The influx to Greece has also sharply dropped since March 20, when a deal between Turkey and the EU came into force that will see migrants sent back -- but the agreement has come under heavy criticism on human rights grounds.

The leaders "urged NATO and the EU to draw on their experience in the Aegean to explore how they could work together to address in an orderly and humane way migrant flows in the central Mediterranean", the White House said.

- Germany seeks EU command -

The naval action envisaged is part of a broader Italian strategy to stop people using Libya as a launchpad for reaching Europe.

This will involve flying migrants with no claim to asylum back to their home countries, which will be paid to set up reception centres to reintegrate them.

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Those plans have been slammed by refugee and rights groups and the EU has also come under fire from Pope Francis for what the Catholic leader sees as an arbitrary distinction between asylum seekers and economic migrants.

Germany has said it supports naval action to combat trafficking of weapons as well as people, but wants it under EU rather than NATO command.

"The USA is fully engaged and ready, in connection with the migration route from Libya to Italy, to share responsibility if necessary," Chancellor Angela Merkel said in Hanover.

"However, we now have a European mission, EUNAVFOR, also called Sophia, which is working quite well."

Aid organisations say over half the boat people arriving in Italy have a clear-cut right to refuge from persecution or conflict and many more deserve proper examination of their asylum applications.

But this year's influx has been overwhelmingly from sub-Saharan Africa, a region the EU considers safe for people's return.

- 'Appalling conditions' -

Under Italy's proposals, an existing NATO mission, Operation Active Endeavour, would be "recalibrated" into one overseeing the Libyan coast.

Any operation would be complicated given the presence of Islamic State fighters in some coastal regions of Libya, but the NATO presence could deter traffickers from putting their human cargoes to sea.

It is unlikely, however, that the operation would seek to turn boats back on the model Australia has adopted in recent years.

"It is worth remembering that Libya is not party to the Geneva convention and that conditions in its detention centres are appalling," said Libya expert Mattia Toaldo.

"I don't think NATO will turn boats back but I do think Italy will start flying people home direct from Sicily."

Any repatriations depend on readmission agreements being concluded with individual countries.

African leaders showed little enthusiasm for that at a summit with their EU counterparts in Malta last year but Brussels' vast aid budget means it has plenty of leverage if needed.

Libya's new national unity government last week offered to sign up to a Turkey-style deal with Italy to take back migrants.

Such an accord had been seen as unlikely because of rights and safety concerns, but Renzi said Monday he did not see why it could not happen.

Italy is preparing to lead a UN-backed peacekeeping force into Libya if the unity government consolidates power sufficiently to be able to ask for outside help without facing a domestic rebellion.