The shocking moment an Australian citizen was pinned down by US immigration agents on the floor of Las Vegas airport has been caught on camera.

A US traveller was walking through terminal three at Harry Reid International Airport on Monday when he saw the incident and started to record.

In footage shared by the bystander on social media, the man, identified as 57-year-old Phu Nguyen, can be heard shouting: 'Let me go!'

'I don't know what they're doing,' Mr Nguyen said. 

A male plain-clothed Immigration and Customs Enforcement (ICE) officer pinned Mr Nguyen's arm behind his back while a female agent placed handcuffs on his other wrist. 

The male agent attempted to hide his face when he saw the bystander filming.

An airport employee then stepped in front of the camera as the two ICE officers quickly walked away, pulling their hoods over their faces.

Mr Nguyen could be seen struggling to get to his feet and collect his backpack, with a handcuff still hanging from one wrist, before police arrived. 

A spokesperson for Australia's Department of Foreign Affairs and Trade (DFAT) told the Daily Mail they were seeking answers on the incident.

'The Department of Foreign Affairs and Trade is making urgent enquiries following reports of an Australian citizen detained in the United States,' the statement said.

'The Department stands ready to provide consular assistance to any Australian citizen, should it be requested.'

The Daily Mail has contacted the US Department of Homeland Security (DHS) for comment.

ICE Los Angeles published a photograph of Mr Nguyen on its X account on Thursday.

'Nguyen overstayed his visa and, despite attempts by agitators to help him evade ICE officers at Las Vegas airport, was taken into custody as soon as he landed in Los Angeles,' it said.

The DHS also confirmed the attempted arrest was carried out by ICE agents and described Mr Nguyen as 'an illegal alien and citizen of Australia' who was born in Vietnam.

'As officers attempted to arrest Nguyen, a crowd of anti-ICE agitators surrounded officers,' a statement said.

In the video, only the bystander could be seen approaching the ICE agents during the attempted arrest.

As the agents walked away, travellers could be heard shouting at them while a woman followed behind. Police then ushered the bystander away after officers arrived at the scene. 

Mr Nguyen is understood to be detained at the Adelanto ICE Processing Centre in California. 

The DHS said he had entered the US legally on a visa in May 2013 with permission to remain until May 2015.

'Nguyen refused to depart in violation of our nation's laws,' the statement said.

'He will receive full due process and remain in ICE custody pending the outcome of his removal proceedings.'