As many as 500 migrants are feared dead after a shipwreck last week in the Mediterranean Sea. If confirmed, it would be one of the worst tragedies of Europe’s migrant crisis in last one year. According to the UN refugee agency UNHCR and the International Organization for Migration, the disaster happened in waters between Italy and Libya. UNHCR team has cited the 41 survivors who were rescued Saturday by a Greek merchant ship.

The incident occurred last week as smugglers tried to move a group of migrants traveling in a boat from the Libyan city of Tobruk to a larger boat crowded with hundreds of people on its way to Italy, according to the UNHCR. A Rome-based spokeswoman for the U.N.’s refugee agency Barbara Molinario said that details of the incident still not clear.