Italy on Friday ignored the pleas of aid agencies and confirmed the end of its search and rescue operation "Mare Nostrum", responsible for saving tens of thousands of boat migrants stranded in the Mediterranean.

"From tomorrow a new operation called 'Triton' begins," deputy prime minister and interior minister Angelino Alfano told journalists at a press conference, referring to a much more limited mission led by the EU border agency Frontex.

"Mare Nostrum ends. Italy has done its duty,"

He said the government had spent some 114 million euros ($142 million) on the massive naval operation since two deadly shipwrecks in October last year left over 400 people dead.

The decision comes after growing criticism both within Italy and Europe that the rescue mission was creating an unintended "pull factor" for more migrants in makeshift boats to attempt the dangerous sea crossings.

Although the op -- which has saved over 150,000 people -- is being shelved, Italy said it will abide by the laws of the sea under which ships that spot migrants in trouble are obliged to rescue them.

Rome's determination to shut down the costly operation has been denounced by aid groups, with the UN High Commissioner for Refugees saying Friday it was "very worried."

"A proper search and rescue effort needs to be maintained, otherwise people will continue to die," UNHCR spokesman William Spindler told reporters in Geneva, adding that the UN agency did not think "Triton" -- which will involve patrols limited to EU waters -- was enough.

The heads of Amnesty International Italy, Dortors without Borders and Italian agency Asgi -- concerned about a possible rise in the number of deaths at sea which have topped 3,300 so far this year -- said in a joint open letter that "Mare Nostrum must not end."

- 'Sea crossings only option' -

"With no safe alternative ways today to seek international protection in Europe, sea crossings are the only option for thousands of people, victims of violence and torture. Disabled people, women and children," the letter said.

"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," it said.

Failing to keep an eye open for boats of migrants and asylum seekers which may be floundering further out to sea means "the risk of seeing other tragedies like the one seen on October 3, 2013, off Lampedusa is very high."

"Mare Nostrum", which is credited with saving an average of 400 people a day, was launched after a fishing boat, overflowing with refugees from east Africa, caught fire and capsized off the Italian island of Lampedusa, killing 366 people in what was described as a wake-up call for Europe.

The agencies warned that the arrival of bad weather as winter approaches "will not put an end to conflicts in Libya, instability in the Sahel, war in Syria and violence in Iraq.... (or) reduce the desperate need to flee war, violence and persecution."

Italy said the answer lay in creating the possibility for would-be refugees to request asylum in their home countries, with Alfano calling on Europe to "change its strategy."

A driving factor behind Italy's decision to end the operation was the soaring cost. The country, struggling to stave off a third recession in six years, was increasingly unwilling to shell out the 9.0 million euros ($11.4 million) a month needed.

Frontex's budget for Triton is more modest, coming in at 3.0 million euros a month, with European Union countries pledging planes and boats for the operation.

"We will do our job in our waters and we will help people if there are requests, according to the laws of the sea. At 30 (nautical) miles from the Italian coast, Europe ends. We'll be there up to that point," Alfano said.

He added that there will a transition phase where Mare Nostrum will accompany Triton for two months.

Defence Minister Roberta Pinotti also promised that Italy "will not turn its back" on those in trouble at sea.