Seventeen-month-old twins were among more than 100 migrants who drowned when their boat deflated off the Libyan coastline earlier this month, according to witness accounts published Tuesday by Medecins Sans Frontier.

The incident only came to light after the aid agency spoke to survivors currently held in detention.

According to witnesses, two rubber boats each carrying around 160 people each set off from the Libyan coastline on Sept 1. One of the boats stopped due to engine failure. The other boat, which was carry around 20 children as well as more than 100 adults, continued but soon began to deflate.

“We called the Italian coastguard and sent our coordinates, asking for assistance as people started to fall in the water,” a survivor told the aid agency. “We were told they'd send someone. But the boat started sinking.”

“We couldn’t swim and only a few people had life jackets. Those among us who could hold on the boat’s floating hood stayed alive,” the survivor added.

Rescue helicopters arrived and dropped life rafts into the water, but “on our boat, only 55 people survived. Many people died," the survivor said.

Sudanese, Malians, Nigerians, Cameroonians, Ghanaians, Libyans, Algerians and Egyptians were among those on board the vessels.

More than 20 children reportedly perished in the crossing. Those that survived were among 276 migrants rescued from several boats that day before being transferred to the port city of Khoms by the Libyan coastguard. They are currently being held in “arbitrary detention,” MSF said.

More than 1,500 people are dead or missing after trying to cross from the North African coast to Europe this year, according to the International Organization for Migration.

Thought the overall number of migrant deaths in 2018 is down from similar figures in recent years, the death rate among those that attempt the crossing has skyrocketed because migrants are taking a much more dangerous route to avoid detection from authorities.

In a statement on its website, MSF said it “reiterates its call to end the arbitrary detention of thousands of refugees and migrants across Libya.”