Thursday's operation in the town of Ellabell was the largest single site raid carried out so far under US President Donald Trump's nationwide anti-migrant drive, a US official said.

Footage of the raid released by US authorities showed detained workers, in handcuffs and with chains around their ankles, being loaded onto an inmate transportation bus.

The raid stemmed from a probe into "allegations of unlawful employment practices and serious federal crimes" at the Hyundai Motor-LG Energy Solution joint venture plant, Steven Schrank, a Homeland Security Investigations special agent in Atlanta, told reporters on Friday.

"This was not an immigration operation where agents went into the premises, rounded up folks and put them on buses," he said. "This has been a multi-month criminal investigation."

South Korean Foreign Minister Cho Hyun said at an emergency meeting in Seoul that, of the 475 arrested, "more than 300 are believed to be our nationals."

"We are deeply concerned and feel a heavy sense of responsibility over this matter," Cho said, adding that he would go to Washington for talks if necessary.

First Vice Foreign Minister Park Yoon-joo raised the issue in a telephone call with US Under Secretary of State for Political Affairs Allison Hooker, voicing regret over the crackdown and the release of footage showing the Korean workers' arrest.

Park said "the economic activities of Korean companies investing in the United States and the rights and interests of Korean citizens must not be unfairly infringed upon during US law enforcement operations," his ministry said.

Park "asked the State Department to actively work to ensure a fair and swift resolution to this matter," the statement added.

Schrank said that those arrested were "illegally present in the United States" and "working unlawfully."

He said those taken into custody have been turned over to Immigration and Customs Enforcement (ICE) for potential removal.

When asked about the raid by reporters at the White House, Trump said: "I would say that they were illegal aliens, and ICE was just doing its job."

The plant where the raid took place is intended to supply batteries for electric vehicles.

LG Energy Solution said Saturday that 47 of its employees had been arrested -- 46 South Koreans and one Indonesian. 

The company also said about 250 of those arrested were believed to be employed by its contractor, and most of them were South Koreans. 

"Business trips to the US will be suspended for the time being unless they are absolutely necessary," the firm's spokeswoman told AFP. 

"Those currently on assignments in the US are going to either return home immediately or remain on standby at their accommodation, taking into account the specifics of their work situation." 

Hyundai said Friday it understood that none of those detained was "directly employed" by the firm.

Schrank said some of those detained had crossed the US border illegally, while others had arrived with visas that prohibited them from working or had overstayed their work visas. 

"This operation underscores our commitment to protecting jobs for Georgians and Americans, ensuring a level playing field for businesses that comply with the law, safeguarding the integrity of our economy and protecting workers from exploitation," he said.

South Korea, Asia's fourth-biggest economy, is a key automaker and electronics producer with multiple plants in the United States.

Its companies have invested billions of dollars to build factories in the United States in a bid to access the US market and avoid tariff threats from Trump.

President Lee Jae Myung met Trump during a visit last month, and Seoul pledged US$350 billion in US investment in July. 

Trump has promised to revive the manufacturing sector in the United States, while also vowing to deport millions of undocumented migrants.