More than 700 migrants are feared dead in three shipwrecks south of Italy in the last few days.
The shipwrecks appear to account for the largest loss of life reported in the Mediterranean since April 2015, when a single ship sank with an estimated 800 people trapped inside.
Carlotta Sami, spokeswoman for UNHCR, said that an estimated 100 people are missing from a smugglers' boat that capsized off the coast of Libya last Wednesday.
The Italian navy took horrific pictures of that capsizing even as it rushed to rescue as many people as possible from the sea.
Sami said about 550 other migrants and refugees are missing from a smuggling boat that capsized on Thursday morning after leaving western Libya a day earlier.
Refugees who saw that boat sink told her agency it was carrying about 670 people, didn't have an engine and was being towed by another packed smuggling boat before it capsized. She said about 25 people from the capsized boat managed to reach the first boat and survive, 79 others were rescued by international patrol boats and 15 bodies were recovered.
Italian police said survivors identified the commander of the boat with the working engine as a 28-year-old Sudanese man, who has been arrested.
In a third shipwreck on Friday, Sami said 135 people were rescued, 45 bodies were recovered and an unknown numbers of migrants were still missing. Because the bodies went missing in the open sea, it is impossible to verify the numbers who died.
Warmer waters and calmer weather of late have only increased the migrants' attempts to reach Europe in unseaworthy smuggling boats, the UN refugee agency said.
Last week, over 4,000 migrants were rescued at sea in one day alone.