Surviving migrants are brought aboard Irish and Italian Navy life-boats in the area where their wooden boat capsized and sank off the coast of Libya Aug 5, 2015. A boat packed with up to 700 African migrants capsized in the Mediterranean Sea off the coast of Libya on Wednesday and many were feared dead.[Photo/Agencies]

VALLETTA -- At least 200 migrants are feared to have died after their boat capsized off the Libyan coast on Wednesday with hundreds of people onboard. Some 400 have been rescued so far.

Hopes of rescuing more survivors are fading as Martin Xuereb, director of the Malta-based Migrant Offshore Aid Station which is involved in the rescue effort, tweeted "it's unlikely that any additional survivors will be picked up."

"Those that were on deck would have managed to jump, some of them drowned and some of them were saved," he added.

Officials initially feared hundreds had drowned but the UN refugee agency said 400 people had been rescued. Some 45 are now confirmed dead, according to Times of Malta's report.

The incident took place some 22 nautical miles north of Zuwarah, a major point of departure for many fleeing Libya.

The boat capsized when migrants attempting the Mediterranean crossing to Italy moved all together towards the same side of the boat to ask for help when they saw a vessel approaching them, Federico Fossi from the United Nations Refugee Agency (UNHCR) told Rai state television.

Wednesday's migrant disaster was likely to be the biggest one since around 800 people drowned in a single accident when their boat sunk in April off the Libyan coast.