Up to 50 refugees and 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 on Wednesday, calling the drownings "shocking and inhumane".

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

At least 22 people are still missing, the IOM said. The passengers' average age was 16, the agency said.

OPINION: Why are African men and women still fleeing to Yemen?

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

The smuggler forced more than 120 people into the sea on 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."

"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

IOM staffers provided aid for 27 survivors who remained on the beach, while others left.

Laurent de Boeck told Al Jazeera that the chaos of Yemen's war is providing fertile ground for people smugglers.

"It's absolutely awful, and this is reflected in the real big business which is happening now in Yemen where there is no capacity to actually control the border. We have seen since the war increased smuggling to the country actually," he said.

"Last year we counted 117,000 people entering the country irregularly - and these are those who have identified," added de Boeck.

'False hope of a better future'

De Boeck called the suffering of refugees and migrants on the route enormous, especially during the current windy season in 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 people 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.

Despite the fighting in Yemen, African refugees and migrants continue to arrive in the war-torn country where there is no central authority to prevent them from travelling onward.

The refugees 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 armed 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 refugees and 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.