MILAN — The feared drowning of 400 migrants in a shipwreck this week in the Mediterranean Sea — one of the deadliest such tragedies in the last decade — raised alarms Wednesday amid an unprecedented wave of migration toward Europe from Africa and the Middle East.

The U.N. refugee agency expressed shock at the scale of the deaths in Monday’s capsizing and renewed calls on European governments to redouble search and rescue efforts, while the International Organization for Migration maintained that the situation had reached “crisis proportions.”

A woman waits to disembark from an Italian Navy vessel in southern Italy after being rescued in the Mediterranean, as workers in protective jumpsuits process incoming migrants.

The Mediterranean “has emerged as the most dangerous” of four major sea routes used by the world’s refugees and migrants, taken by 219,000 people last year, U.N. High Commissioner for Refugees Antonio Guterres said.

The Italian Coast Guard rescued 140 people of the coast of Libya on Monday and recovered nine bodies, but could see immediately from the size of the capsized smuggler’s boat that there had likely been hundreds more on board.

The rescue was made during a five-day surge that saw Italian ships save nearly 10,000 people at sea since Friday — an unprecedented rate in such a short period, according to Cmdr. Filippo Marini, a Coast Guard spokesman. The number is only likely to grow, with summer weather encouraging even more people fleeing poverty and conflict to make the perilous crossing.

Survivors of Monday’s shipwreck reported that as many as 550 people were on board, according to aid workers.

“Of course this is an estimate. No one who travels knows exactly the number. They don’t get a ticket that says: No. 550,” said Barbara Molinario, the U.N. High Commissioner for Refugees spokeswoman in Italy.

Accounts by survivors, mostly sub-Saharan Africans, indicate the ship capsized when men on the upper deck rushed to wave down a ship they believed to be a rescue vessel, said IOM spokesman Joel Millman in Geneva.

“Many were waving and gesticulating to get attention and that caused the vessel to capsize, with the speculation that women and children who were below deck were drowned instantly,” Millman said.

The rescued migrants arrived Wednesday at the southern Italian port of Corigliano, where aid workers dressed in white protective jumpsuits, gloves and masks worked to process them.

A precise accounting of the number of dead will never be known: The search operation was called off after the recovery of just nine bodies due to the depth of the sea, meaning there will be no body count to verify survivors’ accounts, as is nearly always the case.

“For all of these things, we rely on the consistency of the reports we get, but we know these people have been traumatized and through terrible things,” Millman said.

The UNHCR estimates 3,500 migrants died in the Mediterranean last year, up from 600 in 2013. With few bodies recovered, many deaths are never officially confirmed. Instead, their fates are recounted by survivors and, in cases when boats are lost at sea without any rescue attempt, by relatives who report their failure to arrive in Europe.

So far this year, the number of dead or missing at sea is 900, according to the UNHCR, compared with just 17 over the same time last year. Typically, the arrival of migrants making the perilous journey goes up in April as the weather improves, increasing concerns about the coming months.

Overall, since 2000, the IOM estimates that over 22,000 migrants have lost their lives trying to reach Europe, although there are no precise figures.

The U.N. refugee agency is compiling a list of all such incidents since 2011, but Molinario declined to disclose figures because they are just estimates.

She cited a boat with 250 people that departed north Africa in June 2013, according to relative reports, and never arrived in Italy, and the appearance on Libyan shores last August of 15 bodies that led to a search effort that recovered a total of 60 bodies that otherwise would have gone uncounted.

“We take it very seriously if various families call in with the same information: ‘My relatives were on the boat with 300 people.’ When we get 10 or 15 calls with the same data, there is no reason we shouldn’t believe a boat departed on that day,” Molinario said.

Far less common are shipwrecks near shore, like the one near Lampedusa on Oct. 3, 2013, when divers recovered more than 360 bodies. About a week later another boat sank off Malta, with about 200 dead, most never recovered.

“Lampedusa caught the world’s attention because it was the first time it was possible to recover that many bodies, and the reason for that is it happened a few miles from the coast and the water was shallow enough for a recovery operation to occur,” Molinario said.

The twin tragedies focused world outrage and prompted Italy to create a search and rescue operation to patrol the high seas, dubbed Mare Nostrum, or Our Sea. It was scrapped late last year under political pressure in the cash-strapped nation, which was spending $10 million (9.5 million euros) a month on the program. In its place, Europe has responded with border patrols that often take far longer to reach distressed vessels on the high seas.

Monday’s tragedy “only demonstrates how important it is to have a robust rescue-at-sea mechanism in the central Mediterranean,” said Guterres, the U.N. refugee commissioner. “Unfortunately Mare Nostrum was never replaced by an equivalent capacity to rescue people.”

Guterres also called for legal avenues for refugees needing protection to travel to Europe, including emergency visas.

Marini, the Italian Coast Guard spokesman, declined to speculate on what lies ahead for the summer given the improved weather conditions and increased departures of migrants. “We are observing the situation, and we have seen significant numbers (of rescues) in just five or six days,” he said.

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