JOHANNESBURG -- Up to 50 migrants from Somalia and Ethiopia were "deliberately drowned" when a smuggler forced them into the sea off Yemen's coast, the U.N. migration agency said Wednesday, calling the drownings "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, according to the agency.

"The suffering of migrants on this migration route is enormous," says @ldeboeck after latest tragedy in #Yemen ➡️ https://t.co/0NnrcDRKBu pic.twitter.com/MFIJB4c9TX — IOM (@UNmigration) August 9, 2017

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 the oil-rich Gulf countries.

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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.

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 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.

IOM staff assist Somali and Ethiopian migrants who were forced into the sea by smugglers. UN Migration Agency

Despite the fighting in Yemen, African migrants continue to arrive in the war-torn country where there is no central authority to prevent them from traveling onward. The migrants are vulnerable to abuse by armed trafficking rings, many of them believed to be connected to the armed groups involved in the war.

The conflict itself is a deadly risk. In March, Somalia's government blamed the Saudi-led coalition fighting in Yemen for an attack on a boat that killed at least 42 Somali refugees off Yemen's coast.

Some Somalis are desperate to avoid years of chaos at home with attacks by homegrown extremist group al-Shabab and deadly drought. Some Ethiopians have left home after months of deadly anti-government protests and a 10-month state of emergency.

More than 111,500 migrants landed on Yemen's shores last year, up from around 100,000 the year before, according to the Regional Mixed Migration Secretariat, a grouping of international agencies that monitors migration in the area.