ROME (Reuters) - Six migrants died and up to 100 more were missing and feared dead after their rubber boat sank in the Mediterranean on Thursday, the president of the Italian unit of aid group Doctors Without Borders (MSF) said.

A British navy ship rescued 27 people and recovered six bodies about 20 miles from the coast of Libya early on Thursday morning, MSF Italy president Loris De Filippi said.

The new disaster adds to a death toll which aid groups had put at 240 for the three days ending on Wednesday, as migrants continue to leave Libya despite rough seas.

The migrants were transferred to the MSF ship Bourbon Argos, one of several aid agency vessels operating in the area. Survivors told staff that another 90 to 100 people had also been on board and had died.

“It is plausible because these boats are all the same, very long rubber boats that usually carry 120-130 people, completely packed,” De Filippi said.

He said MSF had no information on the nationalities of the migrants rescued on Thursday. Most rescued in recent days were of West African origin.

“The conditions are terrible and people keep putting out to sea anyway, which says a lot about how desperate they are,” he said.

The number of migrants arriving in Italy by boat in November is already more than double that in the same month last year, according to the Interior Ministry, and the total for the full year is fast approaching 2014’s record of 170,000.