US President Joe Biden sharply criticized the forceful treatment of Haitian migrants on the US-Mexico border, saying their treatment was an "embarrassment" for the US while pledging to ensure accountability.

Biden said "it was horrible" to see the scenes of predominantly Haitian migrants forcefully being corralled by border patrol agents, with some pictures that prompted widespread outcry appearing to depict horse-mounted agents using ropes and reins to whip people.

"It’s outrageous," Biden told reporters at the White House, further saying the agents "will pay" for their actions following an investigation. "It’s an embarrassment, but beyond an embarrassment it’s dangerous, it’s wrong. It sends the wrong message around the world. It send the wrong message at home. It’s simply not who we are.”

Biden has faced mounting criticism from fellow Democrats and rights groups for the situation in Del Rio, Texas where thousands of migrants have formed an encampment under a bridge. The administration has already begun the process to deport some of those at the site.

Daniel Foote, Biden's special envoy to Haiti, abruptly resigned Thursday because of what he said was a "deeply flawed" policy on Haiti.

In a letter to Secretary of State Antony Blinken, Foote said he would not be associated with the US' "inhumane, counterproductive decision to deport thousands of Haitian refugees and illegal immigrants to Haiti.

"Our policy approach to Haiti remains deeply flawed, and my policy recommendations have been ignored and dismissed, when not edited to project a narrative different from my own," wrote the ambassador.

The State Department acknowledged Foote's resignation, but flatly rebuffed his claims that his policy suggestions were ignored, saying the suggestion is "simply false" while assailing his decision to leave and how he chose to resign.

"No ideas are ignored, but not all ideas are good ideas," spokesman Ned Price said in a statement.

"This is a challenging moment that requires leadership. It is unfortunate that, instead of participating in a solutions-oriented policy process, Special Envoy Foote has both resigned and mischaracterized the circumstances of his resignation. He failed to take advantage of ample opportunity to raise concerns about migration during his tenure and chose to resign instead," added Price.