Up to 50 migrants from Somalia and Ethiopia were "deliberately drowned" when a smuggler forced them into the sea off Yemen's coast, the UN migration agency said Wednesday, calling the incident "shocking and inhumane."

International Organization for Migration staffers found the shallow graves of 29 of the migrants on a beach in Shabwa during a routine patrol, the agency's statement said. The dead were buried by those who survived.

At least 22 migrants remained missing, the IOM said. The passengers' average age was around 16, the agency said.

The narrow waters between the Horn of Africa and Yemen have been a popular migration route despite Yemen's ongoing conflict. Migrants try to make their way to Gulf countries.

The smuggler forced more than 120 migrants into the sea Wednesday morning as they approached Yemen's coast, the IOM statement said.

"The survivors told our colleagues on the beach that the smuggler pushed them to the sea when he saw some 'authority types' near the coast," said Laurent de Boeck, the IOM's chief of mission in Yemen. "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."

IOM staffers provided aid for 27 surviving migrants who remained on the beach, while other migrants left.

An International Organization for Migration staffer attends to the remains of a deceased migrant on a beach in Yemen. (U.N. Migration Agency)

De Boeck called the suffering of migrants on the route enormous, especially during the current windy season on the Indian Ocean. "Too many young people pay smugglers with the false hope of a better future," he said.

The IOM says about 55,000 migrants, most under 18 years old, have left Horn of Africa nations for Yemen since January, with most from Somalia and Ethiopia. A third of them are estimated to be women.