Christiane Berthiaume, spokeswoman for the organisation, said witnesses claim that the boat left Damietta in Egypt earlier this month with Syrians, Palestinians, Egyptians and Sudanese on board.
It sank last Wednesday after a group of human traffickers deliberately rammed it with another boat when the passengers refused to transfer on to smaller vessels, she added.
News of the shocking incident comes as dozens died when a boat carrying at least 250 Africans to Europe capsized before it even managed to leave the Libyan coast this morning.
Ms Berthiaume said two Palestinian survivors claim there was a violent confrontation between the migrants and traffickers who wanted to move everyone on to one of their smaller vessels.
She says the traffickers subsequently 'used one boat to knock the other' - severely damaging the migrants' vessel and causing it to sink, killing hundreds.
Criminals deliberately rammed boat packed with migrants in Mediterranean
They were angry that passengers refused to move on to smaller vessels
Boat suffered severe damage, causing it to sink and leaving up to 500 dead
News of incident comes as dozens of migrants died off Libya this morning
Boat carrying 250 Africans to Europe capsized just 11 miles from the coast
By JOHN HALL FOR MAILONLINE
PUBLISHED: 09:02 EST, 15 September 2014 | UPDATED: 11:03 EST, 15 September 2014