More than 1,000 migrants and refugees have died in the Mediterranean Sea this year, the sixth year in a row that this "bleak milestone" has been reached, the United Nations said on Tuesday.

The UN refugee agency the UN High Commissioner for Refugees (UNHCR) called for European Union (EU) member states to reactivate search and rescue operations and acknowledge the crucial role of aid groups' vessels in saving lives at sea.

"The tragedy of the Mediterranean cannot be allowed to continue," said Charlie Yaxley, spokesman of UNHCR.

It's reported that the bodies of five migrants were found on Morocco's Atlantic coast near Casablanca on Monday, bringing to 12 the number killed when their boat capsized on Saturday.

The EU struck a deal with Ankara in 2016 to cut off refugee and migrant flows to Greece from Turkey. Departures, now also diverted largely via Libya and other parts of North Africa, have fallen sharply from a peak of more than one million in 2015 to some 78,000 so far this year, UNHCR figures show.

"Of course the number of people attempting to cross the Mediterranean is much lower. So, that points to the fact that the journeys themselves are much more dangerous," said UNHCR spokeswoman Liz Throssell.

"It is also worth highlighting that 70 percent of the deaths actually occur on the central Mediterranean, namely people attempting to get from Libya across to Italy or Malta."

More than 18,000 people have lost their lives in Mediterranean crossings since 2014, according to figures from both the UNHCR and the website of the UN's International Organization for Migration.