Atlanta, Georgia - More than 300 South Koreans were among 475 people arrested by US immigration officials in a raid on a Hyundai-LG battery plant being built in Georgia, the foreign minister in Seoul said on Saturday.

Steven Schrank, a Homeland Security Investigations special agent in Atlanta, earlier said Thursday's operation was the largest single site raid carried out so far under President Donald Trump's nationwide anti-migration campaign.

The raid stemmed from a "criminal investigation into allegations of unlawful employment practices and serious federal crimes" at the Hyundai Motor-LG Energy Solution joint venture plant in the town of Ellabell, Schrank 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."

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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.

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.

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

South Korea had already urged Washington to respect the rights of its citizens before Cho's comments.

"The economic activities of our investors and the legitimate rights and interests of our nationals must not be unjustly infringed in the course of US law enforcement," foreign ministry spokesperson Lee Jae-woong said on Friday.

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, with most of them South Koreans.

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

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"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 without documentation. 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 US. Its companies have invested billions of dollars to build factories in the US in a bid to access the American market and avoid tariff threats from Trump.

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

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