The United Nations says approximately 50 teenage Somali and Ethiopian migrants were "deliberately drowned" by a smuggler who forced passengers into the sea off Yemen's coast.

The International Organisation for Migration (IOM) said survivors reported the smuggler pushed them into the sea when he spotted what he feared were authorities, as his boat neared the coast of Yemen early on Wednesday.

IOM staff found the remains of 29 African migrants buried on the beach in Yemen's Shabwa province along the Gulf of Aden, while survivors reported another 22 passengers missing.

More than 50,000 migrants have reportedly made the journey since the start of the year. ( UN Migration Agency )

A spokesperson for the UN migration agency said the average age of the passengers was 16.

Laurent de Boeck, the IOM Yemen chief of mission, said in a statement survivors reported the smuggler was already continuing his work.

"They also told us that the smuggler has already returned to Somalia to continue his business and pick up more migrants to bring to Yemen on the same route," he said.

Survivors told aid workers that more than 20 migrants were still missing. ( UN Migration Agency )

The migrants, who included some young women, had headed to war-torn Yemen in order to then seek opportunities in Gulf countries.

Already this year, 55,000 migrants from the Horn of Africa have taken the hazardous route, the IOM said.