As many as 500 people are feared dead after a shipwreck last week in the Mediterranean Sea, two international groups said Wednesday, describing survivors' accounts of panicked passengers who desperately tried to stay afloat by jumping between vessels.

The disaster happened in waters between Italy and Libya, based on accounts from 41 survivors who were rescued Saturday by a merchant ship, according to the UN refugee agency and the International Organization for Migration.

The tragedy ranks among the deadliest in recent years on the often-treacherous sea voyage along the central Mediterranean by refugees and migrants from Africa, the Middle East and beyond who have travelled in droves hoping to reach relatively peaceful and wealthy Europe.

'All the people died in a matter of minutes'

While accounts provided by IOM and UNHCR varied slightly, both organizations said up to 200 people left the coastal town of Tobruk last week headed for a larger vessel already carrying hundreds of people in the Mediterranean.

IOM said the 200 people had left on several small boats, while UNHCR said 100 to 200 people left in a single 30-metre boat. The discrepancy in the accounts could not be immediately explained.

UNHCR said the larger boat was already facing "terribly overcrowded conditions" before the newcomers arrived.

Hundreds feared dead after a sinking in Mediterranean Sea between Libya and Italy <a href="https://t.co/Km2zYvwp62">https://t.co/Km2zYvwp62</a> <a href="https://t.co/iEA6Bq4cEy">pic.twitter.com/iEA6Bq4cEy</a> —@Refugees

"Once transferred to the larger vessel — now with an estimated 500 on board — it began taking on water," IOM said, citing survivors' accounts. "The vessel started to sink and panicking passengers tried to jump into the smaller boats they had arrived in."

"The survivors told IOM that most of those aboard the larger vessel tragically died," the agency said in a statement.

It quoted an Ethiopian survivor it identified only as Mohamed as saying: "I saw my wife and my two-month-old child die at sea, together with my brother-in-law. ... The boat was going down ... down. ... All the people died in a matter of minutes."

'I thought I was going to die'

The survivors "drifted at sea for a few days, without food, without anything," Mohamed said, adding that he thought "I was going to die." He said the travellers had intended to go to Italy, not Greece.

In its statement, IOM Athens Chief of Mission Daniel Esdras called the accounts "heartbreaking" and said the organization was awaiting investigations by authorities "to better understand what actually happened and find hopefully evidence against criminal smugglers."

No national authorities in the area have reported any bodies washing ashore. Greek authorities said a cargo ship picked up 41 people on Saturday from a wooden boat that was without steering about 95 nautical miles south of the Greek mainland. The Greek authorities did not describe them as survivors or say anything about any boat sinking.

The survivors were then taken to Kalamata, Greece, where IOM and UNHCR staffers interviewed them. UNHCR said the survivors were 23 Somalis, 11 Ethiopians, six Egyptians and a Sudanese.

Barbara Molinario, a Rome-based spokeswoman for UNHCR, said details remained unclear and said its staffers didn't want to press the survivors too hard "as they are still very tried by their experience."

Refugees disembark from the SOS Mediterranee ship Aquarius at the Lampedusa harbour, Italy, on April 18, 2016. The European Union's border agency says the number of people crossing the Mediterranean Sea to Italy more than doubled last month. Frontex said in a statement on Monday that almost 9,600 refugees and migrants attempted the crossing, one of the most perilous sea voyages for people seeking sanctuary or jobs in Europe. (Elio Desiderio/ANSA via AP) (Elio Desiderio/ANSA via Associated Press)

The statements offered the most official comment yet following repeated news reports about the incident in recent days.

Somalia's president, prime minister and parliamentary speaker issued a joint statement Monday concerning an unconfirmed report about the incident. Reports of the drownings circulated among families and on social media, but they hadn't been confirmed by coast guard authorities in Italy, Greece, Libya or Egypt.

According to IOM's Missing Migrants project, the death toll is the largest from a sinking on the central Mediterranean since another south of Lampedusa in April last year, in which 772 people died. Its largest recorded toll was an October 2013 incident in the same area, when about 800 people died. Several other accidents since then took 400 to 500 lives, its statistics show.

This year, IOM has tallied nearly 800 refugee deaths on the central Mediterranean route and cites reports of another 377 on the eastern route between Turkey and Greece. Five died on the western route between Morocco and Spain this year, the group said.

'Regular pathways,' needed: UNHCR

More than one million migrants and refugees crossed the Mediterranean last year — mostly refugees from war in Afghanistan, Iraq and Syria fleeing to Greece and the European Union, via Turkey. However, the longer Libya-Italy route has traditionally seen more deaths.

Facing internal divisions, the EU has struggled to cope with the influx, and UNHCR on Wednesday reiterated its longstanding call for more "regular pathways" to Europe, such as with resettlement and humanitarian admission programs, family reunification, private sponsorship and student and work visas.

178,882 migrants incl. refugees arrived by sea to Europe 737 dead/missing <a href="https://t.co/Dc8umyVaTU">https://t.co/Dc8umyVaTU</a> <a href="https://twitter.com/hashtag/MigrationEurope?src=hash">#MigrationEurope</a> <a href="https://t.co/w7t2lcmovq">pic.twitter.com/w7t2lcmovq</a> —@IOM_news

Rights groups have repeatedly slammed a new Turkey-EU deal to curtail the flood of refugees into Europe, raising questions about the safety of Syrian refugees on both sides of the Turkish border.

Earlier Wednesday, Human Rights Watch urged Turkey to allow Syrians displaced by government shelling to cross the border to safety. The advocacy groups said the Syrian army hit two migrant camps on April 13 and April 15, triggering an exodus of 3,000 people.

Last week, the rights group said Turkish border guards had shot at Syrians escaping an Islamic State offensive. Turkey, home to 2.7 million Syrian refugees, rejects the claim and says it has an open-door policy toward migrants, but new arrivals are rare.

The rights group says tens of thousands of civilians are trapped along Turkey's border.