A 15-year-old girl in the US has been brutally beaten at a public bus terminal while three security guards stood by and watched.

Footage of the incident was captured on CCTV and shows the victim being jumped from behind by another girl directly in front of the yellow-vested security guards.

The victim can be seen falling to the ground of the Seattle bus tunnel before she is surrounded by other teens who start robbing her of her belongings, ABC World News reports.

"I went to the security and told them that these kids were trying to jump me," the girl told investigators in a statement obtained by the ABC.

"I know that I am about to get jumped and I am hanging around the guards to try and get protection."

"I thought the security guards would defend me."

The teenager who kicked the victim in the head simply walks away from the scene of the attack.

Authorities have launched an investigation into the incident and the policy that requires security guards to "observe and report" instances of violence.

The victim suffered cuts and bruises from the attack and is recovering.