Europeans need to take urgent action to stop this catastrophe getting worse in the second half of 2014," U.N. High Commissioner for Refugees Antonio Guterres said in a statement.
Many of the migrants are fleeing violence in Eritrea and Syria, and most travel from Libya or elsewhere in North Africa.
"Rescued refugees and migrants have reported handing over their life savings to smugglers, in order to travel in unseaworthy and overcrowded dinghies, packed into a few meters of space without food, water or life jackets," the UNHCR statement said.