A South Korean worker who witnessed a massive immigration operation at a car factory in Georgia has told the BBC of panic and confusion as federal agents descended on the site and arrested hundreds.

The man, who asked to remain anonymous, was at the factory which is jointly owned by Hyundai and LG Energy Solution when agents from Immigration and Customs Enforcement (ICE) arrested 475 people, including 300 South Korean nationals, with some being led away in chains.

He said he first became aware of the Thursday morning raid when he and his colleagues received a deluge of phone calls from company bosses. Multiple phone lines were ringing and the message was to shut down operations, he said.

As news spread of the raid, the largest of its kind since President Donald Trump returned to the White House, the man said panicked family members tried to contact the workers.

They were detained and they left all their cell phones in the office. They were getting calls, but we couldn't answer because [the office] was locked, he said.

According to US officials, some workers tried to flee including several who jumped into a nearby sewage pond. They were separated into groups based on nationality and visa status, before being processed and loaded onto multiple coaches.

Some 400 state and federal agents had gathered outside the sprawling $7.6bn factory complex, which is about half an hour from Savannah, before entering the site at around 10:30 on Thursday.

The 3,000-acre complex opened last year and workers there assemble electric vehicles. Immigration officials had been investigating alleged illegal employment practices at an electric vehicle battery plant that is being built in the compound.

The operation ultimately became the largest single-site immigration enforcement operation in the history of Homeland Security investigations, officials said, adding that hundreds of people who were not legally allowed to work in the US were detained.

BBC Verify has been reviewing footage posted on social media and apparently filmed inside the battery plant.

One video shows men lined up in a room as a masked man, wearing a vest with the initials HSI - Homeland Security Investigations - and holding a walkie-talkie, tells them: We're Homeland Security, we have a search warrant for the whole site. We need construction to cease immediately, we need all work to end on the site right now.

The man said he was shocked but not surprised by the immigration operation. He said the vast majority of the workers detained were mechanics installing production lines at the site, and were employed by a contractor.

He also noted that a minority of those arrested had been sent from head office in Seoul and were carrying out training, which the BBC has not been able to confirm.

The man believed nearly all the workers had some legal right to be in the US, but were on the wrong type of visas or their right to work had expired.

The BBC has contacted both Hyundai and LG Energy Solution for comment.

In a joint statement released after the raid, Hyundai and LG Energy Solution said they were co-operating fully with the appropriate authorities regarding activity at our construction site. To assist their work, we have paused construction.

Hyundai also stated that based on our current understanding, none of those detained is directly employed by Hyundai Motor Company.

On Friday, the ICE agent in charge of the operation, Steven Schrank, said all 475 detainees were illegally present in the United States. He elaborated that they were workers who have entered through a variety of different means into the United States, some illegally crossed the border, some came in through visa waiver and were prohibited from working, some had visas and overstayed their visas.

Additional reporting by Aisha Sembhi and Woongbee Lee.