Smugglers have thrown 280 migrants into the sea off the coast of Yemen over the past two days, with close to 70 presumed dead.

In what aid workers described as a case of "deliberate drowning", bodies were found washed ashore and a beach littered with shallow graves as survivors tried to bury the dead.

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The UN's International Organisation for Migration (IOM) said the two disturbing incidents came after smugglers feared they were about to be caught by security forces, and decided to force their passengers overboard.

The migrants were believed to have been crossing the narrow but dangerous waters between the Horn of Africa and Yemen in the hope of reaching oil-rich countries in the Gulf in search of jobs.

On Wednesday (9 August), a human smuggler pushed more than 120 Somali and Ethiopian migrants from his boat into rough seas as they approached the coast of Shabwa, a Yemeni Governorate along the Arabian Sea.

Staff from the IOM found the shallow graves of 29 migrants on a beach in Shabwa during a routine patrol, while urgent care was provided to a further 27 washed-up survivors. Some 22 are still missing and presumed dead.

The average age of the migrants in the boat was just 16, the IOM 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 Yemen Chief of Mission.

"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 following day, 160 Ethiopian migrants were violently forced into the sea off the same stretch of Yemen's coast, the IOM said.

Six bodies – two male and four female – were found washed up on a beach, with 13 still missing. The majority of the migrants left before aid workers arrived, but nearly 60 who stayed behind had to be given emergency medical assistance.

Since January, the IOM estimates that about 55,000 migrants left the Horn of Africa to travel to Yemen, most with the aim of trying to find better opportunities in the Gulf countries.

More than 30,000 were under the age of 18 and from Somalia and Ethiopia, while a third were female.

Survivors from both incidents have described the harrowing journey they faced and the brutal treatment by smugglers.

They told of being forced to squat down during the entire trip from Ambah Shore in Somalia, which sometimes takes between 24-36 hours, so that the smugglers could increase the number of people in the boat.

The migrants were not allowed to move inside the boat and were beaten or killed if they did, they added.

They were also forced to wet themselves due to the lack of sanitary provisions and were not allowed to take enough food or water on the journey.

In some cases, they said smugglers tied their hands, meaning certain death if the boat capsized during the journey.

"The utter disregard for human life by these smugglers, and all human smugglers worldwide, is nothing less than immoral," said William Lacy Swing, IOM Director General.

"What is a teenager's life worth? On this route to the Gulf countries, it can be as little as $100.

"There is something fundamentally wrong with this world if countless numbers of children can be deliberately and ruthlessly drowned in the ocean, when they are no longer an easy source of income, and nothing is done to stop it from ever happening again.

"It should never have happened in the first place. We should not have to wait for tragedies like these to show us that international cooperation must be enhanced to fight human smuggling – not just through policy but through real action along these smuggling routes.

"This is a busy and extremely dangerous smuggling route. Yemen is suffering one of today's most dire humanitarian crises. Countries experiencing conflict or crisis like Yemen need greater support to reinforce law enforcement and humanitarian border management with the aim of protecting vulnerable migrants like these 16-year-old kids."