CAIRO (AP) — Libya's coast guard intercepted Wednesday four boats in the Mediterranean Sea carrying migrants trying to reach Europe, a U.N. official said. According to the migrants, 20 people from one of the vessels had gone overboard earlier in the day and were presumed to have drowned.

The circumstances under which the migrants went overboard were unclear, said Safa Msehli, a spokeswoman for the International Organization for Migration. The vessel was overcrowded as many of the rubber dinghies and dilapidated boats smuggling the migrants typically are.

It was the latest disaster in the Mediterranean involving migrants seeking a better life in Europe.

Since Tuesday, a total of seven vessels smuggling hundreds of migrants were intercepted off Libya's coast, Msehli also said. Around 500 migrants, including nine children and 43 women, were returned to shore and taken to the Mabani detention center in Tripoli, she said.

Many of the migrants were exhausted and suffered from dehydration, she added.

Amnesty International has said that in the first six months of this year, more than 7,000 people intercepted at sea were forcibly returned to detention camps in Libya.

Rights groups and officials at U.N. agencies that work with migrants and refugees have for years now cited survivor testimony about systematic abuse in the camps, including forced labor, beatings, rapes and torture. The abuse often accompanies efforts to extort money from families before migrants are allowed to leave Libya on traffickers’ boats.

Earlier in July, Libyan maritime authorities acknowledged that one of their coast guard vessel had fired warning shots at a migrant boat it was chasing in the Mediterranean, in an apparent effort to stop it from crossing to Europe and endangering the lives of the migrants onboard.