ROME: More than 300 people probably died this week after attempting to reach Italy from Libya on four separate boats in stormy weather, the U.N. refugee agency said on Wednesday after speaking to a handful of survivors.

The sharp increase in the number of migrants dying while trying to cross the Mediterranean has reignited criticism of Italy's decision last year to end a full-scale search-and-rescue mission, known as Mare Nostrum, in order to save costs.

Mare Nostrum was replaced by a European Union border-control mission called Triton, which employs fewer ships and has a much smaller area of operation.

Among the latest incidents, an Italian tug boat rescued nine people, the only known survivors from their two boats, on Monday and brought them to the Italian island of Lampedusa on Wednesday. More than 200 people remain unaccounted for.

The two boats were part of a four-boat flotilla that left a beach near Libya's capital Tripoli on Saturday, Charlotta Sami, spokeswoman for the United Nations High Commissioner for Refugees in Italy, told Reuters.

A third boat, carrying an estimated 100 migrants, is still missing, survivors told the UNHCR. Among the missing, mostly young African men, is a 12-year-old boy.

The Italian coast guard picked up 105 people on Sunday from the fourth boat. The sea conditions were extreme, with waves as high as eight metres (26 feet) and temperatures just a few degrees above zero. Twenty-nine died of hypothermia in the 18 hours it took the coast guard to ferry them to Italy.

Counting the more than 300 estimated to have been in the four missing boats, almost 30 times as many have died since the start of 2015 versus the same period last year, when the Mare Nostrum mission was still in place.

“This is a tragedy on an enormous scale and a stark reminder that more lives could be lost if those seeking safety are left at the mercy of the sea,” Vincent Cochetel, UNHCR Europe Bureau Director, said in a statement.

“Saving lives should be our top priority. Europe cannot afford to do too little too late.” On Tuesday, Italian Interior Minister Angelino Alfano appealed for a “more vigorous” European response to tackle the crisis.

Pope Francis told pilgrims in St. Peter's Square on Wednesday that he was “following with concern the news out of Lampedusa”. He said he would pray for the victims and “again encourage solidarity so that those in need are rescued”.