At least 125 Syrians were on board a migrant ship that sank off Greece this week, relatives and activists said Friday, as rescuers scoured the waters for hundreds feared missing.

A blind 15-year-old boy and his 28-year-old sister from the southern Daraa province were among those on board, said their uncle, who declined to be named for security reasons.

"We have no trace of them, my nephew is blind and does not know how to swim," he said by telephone, adding the pair had flown directly from Damascus to Libya.

"He lived a tough live, hearing the sounds of battles for years. He lived in fear and probably died in fear," he sobbed.

Activists told AFP that most of the Syrians were from the war-torn country's restive south, while a relative said that at least 35 were from Kurdish-held areas in the north.

On Wednesday, a rusty trawler sank off Greece's while en route from Libya to Italy, with at least 78 dead while 104 were rescued.

While the number of people on board at the time remains unclear, the United Nations pointed on Friday to various witness accounts suggesting it was somewhere between 400 and 750.

Activists at the Daraa Martyrs Documentation Office told AFP that at least 90 passengers, 55 of whom are still missing, were from the country's south, mainly from the Daraa province.

"It isn't surprising to find so many youths from Daraa on this boat," said one of the activist, requesting anonymity for security concerns.

"Young people and families have massively fled Daraa over the years because the living and security situation there is absolutely unbearable."

Daraa province was the cradle of the 2011 uprising against Syrian President Bashar al-Assad, but it returned to regime control in 2018.

The brother of a 22-year-old man who was on board the ship that sank told AFP that at least 35 passengers were from the Kurdish-held Kobane region in northern Syria.

His brother was among five people who survived the shipwreck after being trapped in the waters for at least four hours, he said.

"My brother told me that the boat was very overcrowded," said the man, who also did not want to be named for security reasons.

"Six crew members fled in rubber boats and abandoned passengers," he said, quoting his brother who had travelled across Lebanon and Egypt to reach Libya.