The U.N. refugee agency says the shipwreck in the Mediterranean this week, in which 400 migrants are presumed to have died, is among the deadliest single incidents in the last decade.

The tragedy comes amid an unprecedented wave of migration toward Europe from Africa and the Middle East.

UNHCR Italy spokeswoman Barbara Molinari says 900 migrants have died or gone missing at sea so far this year, part of a phenomenon the agency has been tracking since 2011. In most cases, no bodies are found and the deaths are presumed based on survivor accounts or, when no boats are recovered, reports by family members whose loved ones never arrive in Europe.

The UNHCR estimates 3,500 migrants died in the Mediterranean last year, up from 600 in 2013.