South Koreans suspected of working in the United States illegally were the majority of 475 people arrested in a raid on a Hyundai-LG battery plant being built in the southern state of Georgia, a US official said Friday.

Steven Schrank, a Homeland Security Investigations special agent in Atlanta, said the operation was the largest single site raid carried out so far as part of President Donald Trump's nationwide anti-migrant drive.

Thursday's 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.

"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."

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

South Korea expressed "concern and regret" over the raid, and urged Washington to respect the rights of its citizens.

"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," South Korean foreign ministry spokesperson Lee Jae-woong said.

Schrank said the 475 arrested were "illegally present in the United States" and "working unlawfully."

"There was a majority of Korean nationals," he said, adding that it was the "largest single site enforcement operation in the history of Homeland Security Investigations."

In Seoul, a source familiar with the matter told AFP that around 300 South Korean nationals had been detained.

Schrank said he could not give a breakdown of how many of those arrested at the plant, which is intended to supply batteries for electric vehicles, were employed by Hyundai, LG or subcontractors.

Those taken into custody have been turned over to ICE for potential removal, he said.

- Billions in investment -

Schrank said some of those detained had illegally crossed the US border, others 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.

South Korean companies have invested billions of dollars to build factories in America 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 pledged to revive the manufacturing sector in the United States, while also vowing to deport millions of undocumented migrants.

In a statement, Hyundai said 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 added.