At least 1,000 migrants are now believed to have died in a series of shipwrecks that took place off Libya in the last week of May, making it the deadliest week for a year and one of the worst on record.
The tally was revised upward by the International Organisation for Migration (IOM) after the UN Refugee Agency on Sunday put the number at about 700.
Both tallies come from the testimony of survivors.
The IOM said 62 people are confirmed dead and another 971 are missing and presumed dead in nine incidents on the route between Libya and Italy between 25 May and 30 May.
Carlotta Sami, spokeswoman for UNHCR, said on Sunday that 550 migrants are missing from a boat which overturned on Thursday morning after leaving the Libyan port of Sabratha on Wednesday.
She said survivors said that boat, which was carrying about 670 people, did not have an engine and was being towed by another packed smuggling boat before it capsized.
About 25 people from the capsized boat managed to reach the first boat while 79 others were rescued by international patrol boats and 15 bodies were recovered.
In a third shipwreck on Friday, 135 people were rescued, 45 bodies recovered from the water as an unspecified number of others remain missing.
Survivors rescued by European vessels were being taken to the Italian ports of Taranto and Pozzallo. Sami said the UN agency was trying to gather further information.
Italian authorities said European reacue vessels saved 13,000 migrants during this week alone. The surge reflects the high demand that has been created in the past few months after smuggling operations on the coast of Tripolitania were disrupted towards the end of 2015 and early months of 2016.