The United Nations (UN) has reported a second incident of Ethiopian refugees forced off boats by human smugglers near the coast of Yemen.
The migrants had been hoping to reach countries in the Gulf via war-torn Yemen, IOM reported.
He described the forced drownings as "shocking and inhumane".
The International Organization for Migration said Wednesday that up to 50 migrants from Ethiopia and Somalia were "deliberately drowned" by a smuggler off Yemen.
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.
In both cases, most migrants were from Ethiopia and Somalia, the IOM said.
The migrants were escaping the political instability and violence plaguing their nations.
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The IOM estimates some 55,000 migrants have nonetheless left the Horn of Africa for Yemen since the start of 2017, more than half of them under the age of 18.
"They were shocked, exhausted and quite desperate", Laurent de Boeck, the IOM Yemen Chief of Mission, told Reuters in an interview in Brussels. Most of the people on the boat were from Ethiopia and Somalia. 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. The migrants usually make the unsafe journey across the Mediterranean Sea with smugglers, who sometimes overcrowd the boats.
According to IOM, up to 180 migrants were reportedly thrown into the sea from a boat today by the smugglers.
Smugglers on Wednesday forced more than 120 Somali and Ethiopian migrants into rough seas off Yemen to avoid arrest by local authorities, leaving at least 29 dead and 22 missing, the organisation said. It said 84 migrants left the beach before IOM staff arrived while it provided emergency medical assistance as well as food and water to 57 surviving migrants. Twenty-two people are missing.
The average age of those on board was 16.
Yemen is one of a few routes that the migrants use to reach other countries.
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 global agencies that monitors migration in the area.