A US border patrol agent has fatally shot a migrant near the US-Mexico border, US Customs and Border Protection (CBP) said.

The incident took place on Wednesday afternoon near Laredo, Texas in a border town about seven miles away called Rio Bravo. The officer was responding to a call that there was “illegal activity”. CBP has not immediately responded to a request for comment on the nature of the activity that was being reported and to which the officer was responding.

"Initial reports indicate that as the agent attempted to apprehend the group, he came under attack by multiple subjects using blunt objects," the agency said in a statement, referring to the undocumented immigrants as “illegal aliens”.

The officer then fired one shot and killed a young woman in the head, one witness told NBC News. The agency has not released the identity of the officer or the victim.

Marta Martinez, a woman who lives near the scene, posted a video of the incident on Facebook. Though the agency said the officers were investigating activity in a “culvert,” Ms Martinez said there is no culvert on the property. She also noted the group of migrants did not have any objects that looked like they could have been used as weapons against the officer.

The incident occurred in a lot approximately one-quarter mile from the Rio Grande, the river near the Mexican border.

“I didn’t hear any yelling or ‘stop’ or ‘don’t run,’” she told the New York Times about the few minutes before the shot rang out just ten feet from her home. She can be heard screaming at the officer in the video she posted, asking “why did you kill her?” while the officer attempted to do chest compressions on the victim.

US border patrol guards kick over water left for migrants near Mexico border

In the video, officers can be heard saying “See what happens? This is what happens with you people” at the men they captured who appeared to have been with the deceased.

Ms Martinez told a local NBC News affiliate: "They come here to the United States to live the American dream. So there's no American dream right there. She died right there on the scene”.

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The FBI is also investigating the event.

“Last year, there were 17 use-of-force cases with firearms by Border Patrol agents, a decrease from 55 in 2012, according to Customs and Border Protection. The federal agency does not identify how many of the episodes were fatal,” the New York Times reported.