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 the southern US state of 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 US President Donald Trump's nationwide anti-migrant drive.

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."

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 the United States for talks in Washington if necessary.

Schrank said on Friday 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 on Saturday.

"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.

Billions in investment

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

Schrank said some of those detained had crossed the US border illegally, others had arrived with visas that prohibited them from working and others 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 $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.

Hyundai said in a statement it was "closely monitoring" the situation at the Georgia construction site and "working to understand the specific circumstances."

"As of today, it is our understanding that none of those detained is directly employed by Hyundai Motor Company," the firm said.

LG Energy Solution said it was "gathering all relevant details."

"We will fully cooperate with the relevant authorities," it said.

by Chris Lefkow with Claire Lee in Seoul