Survivors who were spotted drifting at sea before being picked up by a passing merchant ship on April 16 told the UNHCR many migrants drowned when human traffickers tried to transfer people on to another already overcrowded vessel somewhere between Libya and Italy.

The latest reported deaths come as Europe struggles to find a way of stemming the flow of people fleeing war poverty and persecution in what has become the continent's worst migrant crisis since World War II.

"The survivors told us that they had been part of a group of between 100 and 200 people who departed last week from a locality near Tobruk in Libya on a 30-metre (100-foot) boat" the UNHCR said in a statement.

"After sailing for several hours the smugglers in charge of the boat attempted to transfer the passengers to a larger ship carrying hundreds of people in terribly overcrowded conditions" it said adding that the larger boat then capsized and sank.

- Arrivals sharply down -

The 41 survivors -- 37 men three women and a three-year-old child -- included migrants who were still on the smaller ship when the one they were being transferred to capsized as well as others who had already boarded the larger ship and managed to swim back when it sank.

They are believed to have drifted at sea for up to three days before being rescued by the Philippine-flagged cargo vessel and taken to Kalamata on the Peloponnese peninsula in southern Greece.

Of those rescued 23 were Somali 11 Ethiopian six Egyptian and one from Sudan the statement said.

One of the Ethiopians said he had lost his wife and child in the sinking according to Zoe Sakouli of the International Organization for Migration (IOM).

The brother of another survivor told AFP in the Somali capital Mogadishu that the boat had left the Egyptian city of Alexandria on April 7.

"Three of my family members died at sea in this tragedy" the man said after speaking to his brother who survived by telephone.

In what is believed to be the deadliest incident involving migrants and refugees trying to reach Europe at least 740 people were feared to have perished in April 2015 after a crammed fishing boat capsized in Libyan waters.

In another incident in September 2014 up to 500 migrants drowned off Malta after people smugglers rammed their boat in an attempt to force them onto a smaller vessel.

The rescued migrants in this week's tragedy were due to arrive in Athens later Wednesday where they will be accommodated in two hotels following several days at a stadium in Kalamata.

The huge refugee influx on European Union shores has sparked fierce disagreements among the bloc's 28 members and brought its system of open borders to the brink of collapse.

Under a new deal with Ankara to try to ease the burden migrants who travel to the Greek islands -- the main entry point for those seeking a new life in Europe -- are being returned to Turkey in return for billions in EU aid.

The EU also promised to resettle one Syrian refugee for every Syrian taken back by Turkey to grant visa-free travel to Turks within the border-free Schengen Zone and to reassess Turkey's stalled EU membership bid.

Migrant arrivals in Greece have fallen sharply since the agreement took effect on March 20.

- Perilous journey -

But about 50000 people remain stranded in Greece since the closure of the migrant route through the Balkans in February.

Over 10000 of them are stuck in a slum-like camp at Idomeni on the border with Macedonia.

More than one million people crossed clandestinely to Europe in 2015 while some 179000 have made the trip since the start of this year according to UNHCR figures.

More than 3700 people died in 2015 trying the make the perilous crossing over the Mediterranean with 761 recorded as dead or missing in 2016.

Authorities fear that calmer seas at the onset of spring will encourage greater numbers of migrants to attempt the perilous crossing to Italy after a winter lull.

On Saturday Pope Francis again highlighted the humanitarian crisis by visiting the Greek island of Lesbos and returning to Rome with three refugee families who fled the conflict in Syria.