CAIRO -- A wooden boat carrying migrants bound for Europe capsized off the coast of Libya and 20 of them drowned, an international humanitarian group said.

The statement from Doctors Without Borders late Thursday came just hours after it was revealed that another shipwreck had claimed the lives of at least 74 migrants in the Mediterranean Sea.

Only three women could be rescued by local fishermen, the group said on Twitter.

"They were in shock and terrified; they saw loved ones disappear beneath the waves, dying in front of their eyes," the statement said.

The wooden boat was carrying 23 migrants who had departed from the Libyan coastal town of Sorman, said Anais Deprade, a spokesperson for MSF, the abbreviation for the French name of the group, Medecins Sans Frontieres.

One of the three survivors lost her husband, her sister and her sister's 1-year-old child, Deprade said.

Alarm Phone, an independent group that supports rescue operations in the Mediterranean Sea, tweeted that the boat had called them for rescue before it sank. The group said it "repeatedly" requested a search-and-rescue operation by EU authorities. "In vain. They decided to let them drown. Our hearts are broken," the group tweeted.

Earlier on Thursday, the UN migration agency said at least 74 migrants had drowned when their boat capsized off the coast of the Libyan port of al-Khums. Only 47 people were rescued by the Libyan coast guard and fishermen and brought to shore. As of late Thursday, 31 bodies were retrieved as the search for the remaining victims continued, the International Organization for Migration said.

In the years since the 2011 uprising that ousted and killed longtime dictator Moammar Gadhafi, war-torn Libya has emerged as the dominant transit point for migrants hoping to get to Europe from Africa and the Middle East. Smugglers often pack desperate families into ill-equipped rubber boats that stall and founder along the perilous Central Mediterranean route. At least 20,000 people have died in those waters since 2014, according to the IOM.

In recent years, the EU has partnered with Libya's coast guard and other local groups to stem the dangerous sea crossings. Rights groups, however, say those policies leave migrants at the mercy of armed groups or confined in squalid detention centres rife with abuses.