"The past few days have been the deadliest, with at least three vessels having overturned or sunk," MENA news agency quoted Melissa Fleming, UNHCR's senior spokesperson, as telling journalists in Geneva.
She said the largest of these incidents occurred last Friday when a boat, reportedly carrying at least 270 people, capsized near Garibouli to the east of the Libyan capital, Tripoli.
Nineteen people were rescued, 100 others bodies recovered and remaining 251 passengers were feared drowned.
The passengers were mainly from Mali, Côte d'Ivoire, Guinea and Sudan.
Maximum number of people who try to enter Europe is Libya, where the security situation has worsened.