Dozens of Somali migrants sailing across the Gulf of Aden are missing and feared dead after one boat capsized and another ran into trouble in rough seas, the U.N. refugee agency said Tuesday.

A smuggling boat carrying 40 Somalis capsized late Saturday as passengers were disembarking off the coast of Yemen about 50 miles east of Mayfa'a, where the U.N. High Commissioner for Refugees operates a reception center, a spokesman said.

Twenty people made it to shore, the rest are missing, William Spindler told reporters in Geneva.

Eight Somalis are known to have died on Sunday afternoon when another smuggling boat carrying 23 passengers hit rough seas some 75 miles east of Mayfa'a, he said. Thirteen made it to shore and two are missing.

"Witnesses said some of the deaths were due to suffocation after the smugglers covered the passenger area with a tarpaulin to prevent water from getting in," Spindler said.

The smugglers who take people across the Gulf of Aden between Somalia and Yemen are notorious for their ruthlessness, such as throwing passengers into shark-infested waters and beating people to death, he said.

More than 17,000 people from the Horn of Africa have arrived in Yemen since the start of the year, Spindler said. Most of them are fleeing violence and poverty in Somalia, he added.

Some 74 people have died attempting the crossing this year alone, Spindler said. A further 51 people are missing.