More refugees and migrants have been "deliberately drowned" by human smugglers for the second time in 24 hours off the coast of Yemen, according to the United Nations migration agency.

The International Organization for Migration (IOM) said in a statement late on Thursday that its staff found six bodies on the beach - two male and four female - and 13 people are still missing.

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.

The UN migration agency said 160 Ethiopian migrants were violently forced into the Arabian Sea on Thursday.

"It is indeed a very dramatic situation," Laurent de Boeck, the agency's chief for Yemen, told Al Jazeera from Brussels on Thursday.

READ MORE: UN says up to 50 refugees down off Yemen

"They were in a boat with smugglers, who dropped them at sea before arriving at the shore. Some people have disappeared. But others were actually buried by their friends on the beaches."

De Boeck said the incident was reported by some of the survivors, whom he described as "exhausted and under shock".

"This situation is new," he said, adding that "it is the first time" that his agency documented people being forced out of the boats by smugglers before reaching the shores.

De Boeck said that the civil war in Yemen and the collapse of the state have allowed "criminal networks to act freely", endangering many refugees and migrants.

Desperate crossing

William Lacy Swing, the head of IOM, lamented the deaths in a video posted on Twitter, saying there was "something fundamentally wrong with this world if countless numbers of children can be deliberately and ruthlessly drowned".

He described the smuggling route from the Horn of Africa to Yemen as "a busy and extremely dangerous route" because of the conflict there.

The war between the Saudi-backed government and Iran-backed Houthi rebels has killed more than 10,000 people and wounded a further 44,500 since 2015.

"We condemn the acts of smugglers off the coast of #Yemen." A message from our @IOMchief Swing: pic.twitter.com/lziQOzVmC5 — IOM (@UNmigration) August 10, 2017

In the incident on Wednesday, the smuggler forced more than 120 people into the sea as they approached Yemen's coast, the IOM statement said. Its staffers found the shallow graves of 29 migrants on a beach in Shabwa during a routine patrol. At least 22 migrants remained missing.

The passengers' average age was 16, the agency said.

De Boeck told Al Jazeera on Thursday that smugglers from Djibouti, Ethiopia and Somalia work with their counterparts in Yemen to transport people across the Horn of Africa.

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.

But once in Yemen, the refugees and migrants face the brutality of war, forcing them back to Sudan, Egypt and Libya, where they cross the Mediterranean to reach Europe.

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.

Since January this year, the IOM said that about 55,000 people had left Horn of Africa nations for Yemen. A third of them are estimated to be women.

Stephane Dujarric, spokesperson for the UN Secretary General, told reporters on Thursday that the international community must give priority to preventing and resolving situations "which both generate mass movement and expose those already on the move to significant danger".

"We must also increase legal pathways for regular migration and offer a credible alternative to these dangerous crossings for people in need of international protection," he added.

Al Jazeera's Rosiland Jordan, reporting from the UN in New York, said many of the refugees had been handcuffed by smugglers which - according to survivors - had been done to get more pepole into the boats and to make it impossible for them to escape and report on the smugglers if something went wrong.

Jordan said it was unlikely the smugglers could be arrested, detained, or tried given that Yemen is in a state of war.

"So there's a real sense of urgency [at the UN] in trying to prevent this from happening again, but with Yemen in a state of war, and its security forces really outmanned by these smuggling operations, this situation could get worse before it gets better," she said.