About 170 people are feared to have died in two separate Mediterranean shipwrecks, the UNHCR says.
The Italian navy reports a ship sank off the coast of Libya with 117 people on board, while Moroccan and Spanish authorities have tried to find a lost boat in the western Mediterranean.
The UN's refugee agency could not independently verify the death tolls.
More than 2,200 people lost their lives trying to cross the Mediterranean in 2018.
"We cannot turn a blind eye to the high numbers of people dying on Europe's doorstep," UN refugee high commissioner Filippo Grandi said in a press release.
"No effort should be spared, or prevented, from saving lives in distress at sea."
The first boat reportedly disappeared with 53 people on board in the Alborán Sea at the western end of the Mediterranean.
One survivor is being treated in Morocco after spending 24 hours stranded at sea.
A search for the vessel over several days has so far been unsuccessful.
The second ship, a dinghy, left Libya on Saturday, according to the International Organisation for Migration (IOM).
Spokesman Flavio Di Giacomo said three survivors told them 120 people were on board when it departed Garabulli in Libya.
An Italian airforce plane dropped two rafts to the boat on Friday after seeing it sinking in rough waters, Navy Rear Admr Fabio Agostini told broadcaster RaiNews24.
Three people suffering severe hypothermia were pulled from the waters by a helicopter, and are being treated on the island of Lampedusa, he said.
The IOM says 4,216 migrants have crossed to Europe by sea in the first 16 days of 2019 - more than double the number arriving in the same period last year.
Several European nations - including Italy - have resisted accepting migrants in recent years.
Italy's populist deputy PM Matteo Salvini wrote on Facebook after the sinking that "as long as European ports remain open... unfortunately the smugglers will continue to do business and kill."