Amnesty International on Thursday warned European leaders against re-deploying warships used to rescue migrants to instead take on people-smuggling gangs, saying thousands of lives had been saved in the past months.

The London-based human rights watchdog hailed a decision at a European Union summit in April to send new naval resources for search and rescue missions.

But it warned that some of the warships may now be used to take part in an EU military operation that began last month intended to curb human trafficking.

"The implementation of this operation should not distract assets from the essential task of patrolling the high seas with a view to coming to the rescue of people in distress," Amnesty said in a statement.

"People will continue to die and large-scale search and rescue operations remain necessary for so long as high numbers continue to take this dangerous route."

Amnesty cited figures showing that one in 16 people attempting the sea crossing died in the first four months of 2015, prior to the emergency measures being agreed by EU leaders.

For May and June, that proportion had fallen to one in 427 people, the organisation said.

"Any withdrawal of resources, or diversion of their use, will jeopardise the current success in saving lives," it said.

The United Nations refugee chief has also warned the European Union about its military operation, saying migrants attempting the risky sea crossing from north African shores must be protected.

"Our position has been very clear: first priority rescue at sea -- lives need to be saved, nobody should be left to die in the Mediterranean," UN High Commissioner for Refugees Antonio Guterres has said.

The new military mission, dubbed "EU NAVFOR Med", will initially involve five warships, two submarines, three maritime patrol aircraft, drones and helicopters.

Nearly 150,000 people have made the perilous journey across the Mediterranean to Europe so far this year -- half arriving in Italy and another half in Greece, according to figures from the International Maritime Organization.