About 500 migrants are feared dead after their ship was rammed by another boat near Malta last week, a migration body said.
Two Palestinian survivors told the International Organisation for Migration (IOM) that the boat had been intentionally sunk by traffickers.
They said the boat had left Damietta in Egypt in early September.
The IOM says that more than 2,500 people are now believed to have drowned in the Mediterranean this year.
News of the sinking near Malta emerged as another vessel carrying 250 people sank off the coast of Libya.
Over 200 people are feared to have drowned in that incident.
IOM spokeswoman Christiane Berthiaume said that the two survivors from the Malta sinking were rescued on Thursday, the day after their boat sank.
They said traffickers rammed the boat after a "violent confrontation" on board. The IOM said there were nine known survivors in total.
The boat had been carrying Syrians, Palestinians, Egyptians and Sudanese, the survivors said.
The passengers, who included women and children, were reportedly told to move to a smaller, less safe boat. When they refused, the traffickers sank the larger vessel, the eyewitnesses said.
The Maltese authorities have not yet commented on the incident.
The UN says more than 130,000 migrants have arrived in Europe by sea this year, compared with 80,000 last year. Italy has received more than 118,000 migrants, the UN said.
Many attempt to cross from North Africa and the Middle East in unsafe and overcrowded vessels.
The UN's Andrej Mahecic told the BBC that more than half of those arriving by boat were refugees from Syria and Eritrea.
Major migrant boat tragedies
- March 2009: More than 200 African migrants drown after their boat sank off the coast of Libya
- October 2013: 366 people, mostly Eritrean, die when their boat catches fire and sinks near Lampedusa
- August 2014: Around 170 feared dead after another boat sinks off Libya