European aid groups have warned of an imminent surge in migrant deaths in the Mediterranean as Italy prepares to phase out its year-long "Mare Nostrum" search and rescue operation and replace it with a more limited mission headed by a European agency.

Mare Nostrum — Latin for "our sea" — was launched last October in response to two catastrophic boat incidents that killed over 360 people, mostly Eritreans. It has since saved the lives of tens of thousands of desperate migrants seeking to cross into Europe from North Africa and the Middle East.

The operation is largely credited with saving some 400 lives a day, but at a high cost: roughly 114 million euros ($142 million) per year. Critics have also said the program leaves Italy open to illegal immigration and encourages smuggling.

On Friday, Italy's deputy prime minister and interior minister, Angelino Alfano, announced the government would shelve the costly naval operation. Instead, the EU border agency Frontex will step up to coordinate a new "Triton" border surveillance mission scaled back to a cost of 2.9 million euro ($3.6 million) a month, and restricted to activity within 30 nautical miles off Italy's coast.

"From tomorrow a new operation called 'Triton' begins," Alfano said at a press conference. "Mare Nostrum ends. Italy has done its duty."

The Italian navy will maintain a presence on the Mediterranean over the next two months as the transition to Triton takes place, Alfano said. Italy also denied any "abdication from rescue duties," and said that it would continue to meet its international obligations to assist any vessel in distress.

Human rights groups voiced concerns this week that the shuttering of Mare Nostrum would lead to more deaths at sea, adding to the some 3,300 people who have drowned since January in the Mediterranean, many near the coast of the Sicilian island of Lampedusa.

The UN High Commissioner for Refugees said Friday it was "very worried" and that Triton may not be adequate enough to prevent tragedy.

'A proper search and rescue effort needs to be maintained, otherwise people will continue to die.'

"A proper search and rescue effort needs to be maintained, otherwise people will continue to die," UNHCR spokesman William Spindler told reporters in Geneva.

A wave of conflicts across North Africa and the Middle East has led to an uptick in migrants crossing over the dangerous waters into Europe this year. Most journey in overcrowded and unseaworthy boats, and many die at sea before reaching their destination, according to the UN.

Even so, with few other options in their homelands, more migrants than ever are making the journey. The Italian interior ministry said more than 153,000 people made it to Italian shores in 2014, which is more than twice the number recorded last year.

The heads of Amnesty International Italy, Doctors without Borders, and the Italian agency Asgi wrote a a joint open letter published in La Republica Friday saying, "Mare Nostrum must not end."

"Search and rescue missions limited to Italy's territorial waters put at risk thousands of lives if the areas of open sea are not actively patrolled," the letter said.

"There are no alternative routes to reach Europe, and conflicts are on the rise not only in Syria but also Iraq and other places. So there will be a greater risk of more sea tragedies without Mare Nostrum," Stefano Di Carlo, Doctors Without Borders operational chief in Italy, told Reuters.

Italy has long appealed for more European assistance in dealing with the migration issue, as it says its borders also serves as a gateway into other countries. It has also called on Europe to "change its strategy," on refugees, and allow people to request asylum from their home countries to help stem the boat problem.

The 21-nation Triton mission is the first step to Europe meeting Italy's demands. Alfano also said the new system would shift the burden of responsibility for search and rescue missions back to northern African nations such as Egypt and Libya.

The worsening migrant situation was reinforced on Thursday after another refugee-laden vessel capsized after leaving Libya. At least 20 are feared dead, according to the Guardian.

Up to 500 migrants, including 100 children, died trying to reach Italy in late September — reportedly after their smugglers deliberately sunk the boat, following a confrontation onboard.

VICE News' Alice Speri contributed to this report.