The boat sank last week but it is believed up to 500 may have drowned

to transfer them onto another boat

But they stopped in the

The number of migrants who died when an overcrowded boat sank of the coast of Libya last week is believed to be as high as 500, the UN says.

Witnesses say the people smugglers set off from North Africa with between 100 and 200 people on board before stopping in the sea and transferring them onto a larger boat.

Survivors found at sea on April 16 told rescuers in Greece they believed about 500 people had gone overboard.

Survivors told rescuers the boat (not pictured) carrying them from Libya had about 500 people on board when it sank

The 41 survivors drifted at sea before being spotted and arrived the following day in Kalamata, in Greece, the UN refugee agency said.

They recounted that they had been among 100 to 200 people who set sail from Libya last week headed for Italy.

After several hours at sea, the traffickers tried to move them onto a bigger ship that was already packed with migrants.

This ship sank before the survivors could board it. They then drifted at sea for up to three days before being saved.

The UN said the date of the sinking was still unclear, and gave no information about who rescued the migrants.

UNHCR spokeswoman Carlotta Sami said survivors described seeing 'a large shipwreck that took place in the Mediterranean Sea claiming the lives of approximately 500 people'.

The survivors - 37 men, three women and a three-year-old child from from Somalia, Sudan, Ethiopia and Egypt - said they had set off from near Tobruk in Libya in a large, unseaworthy vessel.