The UN migration agency says up to 50 migrants from Somalia and Ethiopia have been "deliberately drowned" when a smuggler forced them into the sea off Yemen's coast.
The International Organisation for Migration (IOM) said a human trafficker in charge of the boat forced more than 120 migrants into the sea as they approached the coast of Shabwa along the Arabian Sea.
Staff from the IOM said they found the shallow graves of 29 of the migrants on a beach in Shabwa during a routine patrol.
Medical staff said they provided urgent care for 27 surviving migrants who remained on the beach. while other migrants had already left. The agency said 22 people are still unaccounted for.
The average age of the passengers on the boat is believed to have been 16-years-old, according to the UN agency.
The smuggler allegedly pushed the migrants into the water when they saw a boat belonging to authorities approaching.
The dead had been buried rapidly by those who survived and the IOM said it it working closely with the International Committee of the Red Cross to ensure appropriate care is given for the deceased migrants' remains.
Laurent de Boeck, the IOM chief of mission in Yemen, called the incident "shocking and inhumane."
"The survivors told our colleagues on the beach that the smuggler pushed them into the sea, when he saw some 'authority types' near the coast.
"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.
"This is shocking and inhumane. The suffering of migrants on this migration route is enormous. Too many young people pay smugglers with the false hope of a better future,"
The narrow waters between the Horn of Africa and Yemen have been a popular migration route despite Yemen's ongoing conflict.
Since January this year the IOM estimates that around 55,000 migrants left the Horn of Africa to come to Yemen with the aim to try and find better opportunities in the Gulf countries.
According to the IOM, more than 30,000 of those migrants are under the age of 18 from Somalia and Ethiopia and a third are estimated to be women.
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