Three young sisters have drowned in a rubber boat carrying migrants in the central Mediterranean after waves of up to 1.5m (4.9 ft) repeatedly washed over the vessel, a German rescue charity has said.

About 65 people were rescued, including the sisters' mother and brother, as well as three pregnant women and a seven-month-old baby, RESQSHIP added.

The "dangerously overcrowded" rubber boat had departed from Libya's Zuwara city and "started taking on increasing amounts of water" a few hours later, the charity said.

Libya is a major launching pad for migrants who make the treacherous journey across the Mediterranean, hoping to reach the Italian island of Lampedusa.

The sisters who drowned were aged nine, 11 and 17 years, RESQSHIP said in a statement.

Recalling how their bodies were discovered early on Saturday, Barbara Sartore, the charity's communications coordinator onboard the rescue ship Nadir, said that while survivors were being evacuated one by one, she heard "screams and someone pointed to the water inside the boat".

"It became clear that there were bodies underneath the surface," Ms Sartore said.

"The boat was dangerously overcrowded, it was pitch-dark, water was flooding in, people were panicking. In that chaos, it was impossible to see that the three sisters, sitting deep inside the boat, had already drowned. When the survivors realised, it was sheer horror," she added.

RESQSHIP said that many of the survivors suffered severe chemical burns caused by the mixture of seawater and petrol inside the boat, and required medical treatment.

One person who had earlier fallen overboard was still missing, the charity added.

Italy's coastguard evacuated 14 people on Saturday afternoon and took them to Lampedusa, while the rescue ship arrived later with the rest of the survivors and the bodies of the girls.

"What happened to the three sisters is unimaginable, as is the danger that people on the move face in search of safety," said Katja Schnitzer, a crewmember of the rescue boat.

The charity did not give the nationalities of the girls or the survivors.

UN agencies say that more than 700 people have died trying to cross the central Mediterranean from North Africa this year.

They say that search and rescue operations need to increase, and the safety of survivors guaranteed when they disembark.