More than 600 migrants have been rescued in the Mediterranean as they attempted to make the perilous crossing to Spain, the Spanish coastguard has said.

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.

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 said.

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 Organisation for Migration - more than during all of last year.

It comes as the UN refugee agency has said that crossing the Mediterranean has become more treacherous than ever for migrants trying to reach Europe, with one in 18 dying or going missing - more than double the rate last year.

More than 1,600 people have died or vanished en route to Europe this year, mostly while attempting to cross by sea from northern Africa, according to a UNHCR report.

The publication of the report, "Desperate Journeys", coincides with the third anniversary of the death of Alan Kurdi, a Syrian refugee boy whose body washed up on a Turkish beach at the height of the migrant crisis, sparking global outrage.

Although arrivals have plummeted in recent years, European countries remain bitterly divided over how to share the burden of refugees and migrants fleeing war, persecution and poverty in the Middle East, Africa and Asia.

"This report once again confirms the Mediterranean as one of the world's deadliest sea crossings," said Pascale Moreau, Director of the UNHCR's Europe Bureau.

"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."

The UNHCR said about 72,000 people had arrived in Italy, Greece and Spain between January and July, compared to about 121,000 for the same period in 2017.

More than one million arrived in 2015.

However, one in 18 who attempted the risky central Mediterranean route died or went missing, up from one in 42 in the first part of last year.

The top countries of origin this year are Syria, Iraq and Guinea. Last year they were Nigeria, Guinea and Ivory Coast.

The report said a key reason for the increased death rate was the reduced search and rescue capacity off the Libyan coast.

Non-governmental organisations (NGOs) that operated there last year have largely been replaced by the Libyan Coast Guard.

The report said rescues often happened further out to sea, meaning refugees were travelling for longer on unsafe boats.

Italy's new populist government has meanwhile refused to let a number of NGO rescue vessels dock and is demanding the European Union find other ports to disembark migrants.

The UNHCR urged European countries to ensure a predictable, regional approach for rescues and disembarkation.

It also called for states to increase resettlement places for refugees and remove obstacles to family reunification so that people do not risk their lives at sea.

Overall, arrivals in Italy fell in the first seven months of 2018, compared to the same period last year, while arrivals rose in Greece and Spain, now the primary entry point to Europe.