"UNHCR is urgently calling for an increase in search and rescue capacity, including a return of EU state vessels to search and rescue operations, and an acknowledgement of the crucial role of NGO boats in saving lives at sea," UNHCR spokesman Charlie Yaxley said in a statement on Tuesday.
At least 15,000 people have lost their lives in Mediterranean crossings since 2014, the UN's International Organization for Migration (IOM) said.