MADRID - More than 600 migrants were rescued in the Mediterranean on Monday as they attempted to make the perilous crossing to Spain, the coastguard said.

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While the overall number of migrants reaching Europe by sea is down from a peak in 2015, Spain has seen a steady increase in arrivals this year and has overtaken Italy as the preferred destination for people desperate to reach the continent.

On Monday, 626 people from sub-Saharan and north Africa were rescued from 16 boats in the Strait of Gibraltar and the Alboran Sea, the two principal sea crossings to Spain, a spokesman for the Spanish coastguard told AFP.

He said rescue workers were also on their way to save another boat in distress, but he did not know how many migrants were on board.

More than 28,500 people have arrived in Spain by sea so far this year, with 313 dying in the attempt, according to the International Organization for Migration - more than during all of last year.

Even as the number of migrants and refugees crossing the Mediterranean to Europe has fallen sharply, the likelihood of dying during the dangerous voyage has risen significantly, the UN said Monday.

Between January and July, more than 58,000 asylum seekers and migrants reached Europe's shores after crossing the Mediterranean Sea - 41 percent fewer than during the same period in 2017, the UN refugee agency said. The number of people taking the particularly dangerous central Mediterranean route to Italy fell to just 18,500, down from over 95,000 during the first seven months of last year, the United Nations High Commissioner for Refugees (UNHCR) said in a fresh report.

But the agency warned that Mediterranean crossings were now "deadlier than ever", with the rate of deaths at sea rising sharply.

During the first seven months of the year, nearly 1,600 people perished trying to make the treacherous journey.

Around 1,200 of those deaths occurred along the central Mediterranean route - around half the number seen last year when there were five times as many arrivals in Italy.

That means that one person died for every 18 who attempted to make the journey along that route, compared with one in 42 who crossed during the same period in 2017.

"This report once again confirms the Mediterranean as one of the world's deadliest sea crossings," Pascale Moreau, who heads UNHCR's Europe operations, said in a statement.

"With the number of people arriving on European shores falling, this is no longer a test of whether Europe can manage the numbers, but whether Europe can muster the humanity to save lives," he said.

Despite the dramatic drop in arrivals, Europe is facing a large-scale political crisis over the migrants and refugees who continue to arrive from Africa and the Middle East.

Italy's recently-installed populist government has vowed to stop taking in migrants rescued off Libya, and along with Malta has repeatedly closed its ports to NGO ships crisscrossing the Mediterranean to help those in need.

Spain, which has opened its ports to several rescue ships run by charities which were turned away from Italy, has meanwhile seen the number of arrivals during the first seven months of this year more than double to 27,600, UNHCR said.

In recent months, UNHCR and the International Organization for Migration have been calling for a "predictable, regional approach" to the rescue and disembarkation of people in distress in the Mediterranean.

Monday's report was released three years after photos of the limp, lifeless body of Syrian boy Alan Kurdi washed up on Turkish shores brought into sharp relief the dangers faced by migrants and refugees trying to reach safety in Europe.

Afghan author and UNHCR Goodwill Ambassador Khaled Hosseini warned that Europe had not heeded the lessons present in those shocking pictures.

"When I saw those devastating images of the body of Alan Kurdi, my heart shattered," Hosseini said in the statement.

"Yet, just three years on and despite thousands more people losing their lives at sea, our collective memory and urgency to do better seems to have faded."