Amnesty International is urging European nations to launch a massive humanitarian operation in the Mediterranean where hundreds of migrants are feared to have drowned in recent weeks.

The human rights group called in a report Wednesday for a multinational effort backed by "naval and aerial resources at a scale commensurate with foreseeable departure trends."

The report said almost 21,000 people had crossed the central Mediterranean this year to April, almost the same number as last year, but that far more had died or disappeared.

European Union leaders are holding crisis talks on migration in Brussels on Thursday.

Amnesty International urged them to send emergency financial and logistical aid to front-line countries Italy and Malta to boost their search and rescue capacity until a multinational operation can be launched.