Almost 500 refugees died when an overcrowded migrant ship sank in the Mediterranean last week, the UNHCR has said.

Citing eye-witnesses, the UN agency officials said hundreds of people drowned in a matter of minutes as the boat sank.

The latest eye-witness accounts confirmed fears that hundreds might have perished in the migrant boat capsize over the weekend.

"We don't know exactly how many were there on that boat and they have now disappeared from the face of the earth. This is another example of what is happening almost in a daily basis in the Mediterranean," UNHCR spokesperson William Spindler told Reuters television.

As many as 41 survivors, who drifted on waters for a few days, were rescued by a Philippine merchant ship that brought them to the Greek port of Kalamata.

"The boat was going down, down. All the people died in a matter of minutes. After the shipwreck we were drifted at sea for a few days, without food, without anything," an Ethiopian man told the International Organization for Migration (IOM).

The man's wife, two-month-old child and brother-in-law drowned after the boat capsize.

Around 200 migrants from various African countries were travelling to Italy from Libyan coastal town of Tobruk. The migrants said they were transferred midway to a larger vessel that was already carrying more than 300 migrants.

"The traffickers made us get on to a bigger wooden boat around 30 metres in length that already had at least 300 people in it," a Somali refugee who escaped the ordeal said.

According to the UN data at least 800 people have died at sea trying to cross over to Europe so far this year alone. Italian officials say around 25,000 migrants have reached the country this so far this year.

A year earlier, almost 800 migrants drowned off the Libyan coast after their boat collided with a merchant ship that was trying to rescue them.

The UN agency said last week's disaster could be the worst maritime incident in the last 12 months.