image source, AFP

Images which appear to show border agents on horseback driving migrants back to a river like cattle have sparked an investigation in the US.

The pictures widely shared on social media show the riders using their reins against the migrants and pushing them back towards the Rio Grande in Texas.

Homeland Security Secretary Alejandro Mayorkas says his department will investigate reports of alleged abuse.

He said the officers were trying to manage the migrants crossing the river.

Some 13,000 mainly Haitian migrants have gathered in a makeshift camp under a bridge connecting Del Rio to Mexico's Ciudad Acuña on the US-Mexico border.

White House spokeswoman Jen Psaki said the footage was horrible to watch.

"I have seen some of the footage. I don't have the full context. I can't imagine what context would make that appropriate. But I don't have additional details and certainly I don't think anyone seeing that footage would think it was acceptable or appropriate."

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Some social media users said the pictures were reminiscent of the violence seen in slavery times in the US.

The pictures taken by AFP photographer Paul Ratje show Haitians going back and forth across the border to get food for their families, and finding themselves blocked by the horses.

"Some of the migrants started running to try to get around the horsemen, and one of the agents grabbed the Haitian in the picture by the shirt and he ended up swinging him around while the horse trotted in a circle," he said of a couple of particular photographs.

image source, AFP

Ratje says he does not think the man was hurt; shortly after that, he said they "kind of calmed down, and they started letting people in".

image source, AFP

US Border Patrol chief Raul Ortiz said the incident was being investigated to make sure there was not an "unacceptable" response by his officers, adding they operated in a difficult environment, trying to distinguish migrants from smugglers.

image source, AFPimage captionUS Border Patrol agents on horseback have been trying to close off crossing points along the Rio Grande river

The UN's refugee agency, the UNHCR, has said it is monitoring US expulsions to Haiti, adding it was imperative that people fearing persecution had access to asylum, Reuters news agency reports.

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The US government has reported a surge of migrants arriving at the border with Mexico this year.

The number of people detained there in July exceeded 200,000 for the first time in 21 years, according to government data.