By Hyunjoo Jin, Heekyong Yang and Ted Hesson

ATLANTA/WASHINGTON/SEOUL (Reuters) -Hundreds of workers at a Hyundai Motor car battery facility under construction in Georgia were detained in a major raid by U.S. authorities late on Thursday, stopping work on a plant that is one of the Korean automaker's major investments in the U.S.

About 475 workers were arrested, according to U.S. immigration officials, the largest single-site enforcement operation in the U.S. Department of Homeland Security's (DHS) history.

President Donald Trump's administration has been escalating a crackdown on immigrants, disrupting businesses around the country, even as the White House has encouraged more inflows from foreign investors.

The arrests could exacerbate tensions between Washington and Seoul, a key ally and investor in the U.S. The countries have been at odds over the details of a trade deal that includes $350 billion of investments. At a summit last month, South Korea pledged $150 billion in U.S. investments - including $26 billion from Hyundai Motor.

Homeland Security officials said the workers arrested at the Ellabell, Georgia site were barred from working in the U.S. after crossing the border illegally or overstaying visas. The investigation took place over several months, Steven Schrank, special agent in charge of investigations for Georgia, said during a press briefing.

"This was not an immigration operation where agents went into the premises, rounded up folks and put them on buses," he said. Schrank said there was a network of subcontractors on the site; a Hyundai Motor spokesperson said none of the people detained were employed directly by the automaker, which complies "with all laws and regulations wherever we operate."

The workers were being held at ICE's Folkston, Georgia detention facility, Schrank said. Most of the 475 people are Korean nationals, he said. Local Korean media said roughly 300 people detained were South Korean nationals.

A spokesperson at Hyundai's battery joint venture partner, South Korean battery maker LG Energy Solutions, said in a statement it was cooperating and had paused construction work. The facility, a joint venture between LGES and Hyundai Motor, was due to start operations at the end of this year, according to LGES.

LGES shares dropped 2.3%.

Under Trump, Immigration and Customs Enforcement, or ICE, part of DHS, has driven the Republican leader's sweeping crackdown on migrants, bolstered by record funding and new latitude to conduct raids.

Trump has said he wants to deport "the worst of the worst" criminals but ICE figures have shown a rise in non-criminals being picked up. Rights advocates have denounced such raids.

The White House said Friday that "any foreign workers brought in for specific projects must enter the United States legally and with proper work authorizations."

South Korea's Foreign Ministry expressed regret and concern about the raid. "The economic activities of our companies investing in the United States and the interests of our citizens must not be unduly violated during the course of U.S. law enforcement," ministry spokesperson Lee Jae-woong said in a statement on Friday.

Social media video showed a man wearing a vest with the letters HSI, an acronym for Homeland Security Investigations, telling workers in yellow safety vests: "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 U.S. Department of Justice in a statement said several people tried to flee during the raid. Some had to be fished out of a sewage pond on the site, DOJ said.

Georgia's Democratic Party condemned the raid, calling it part of "politically-motivated fear tactics designed to terrorize people who work hard for a living, power our economy, and contribute to the communities across Georgia that they have made their homes." In a statement, a spokesman for Georgia Governor Brian Kemp said: “In Georgia, we will always enforce the law, including all state and federal immigration laws."

Hyundai said its production of electric vehicles at the sprawling site was not affected.

In 2023, Hyundai Motor and LG Energy announced the $4.3 billion venture to produce EV battery cells, with each company holding a 50% stake. The plant will supply batteries for Hyundai, Kia and Genesis EV models.

The battery factory is part of Hyundai's $12.6 billion investments in the state, including the automaker's just-opened car factory, in what would be "the largest economic development project in the state’s history."

(Reporting by Hyunjoo Jin and Heekyong Yang in Seoul and Ted Hesson in Washington; additional reporting by Trevor Hunnicutt and Kaniskha Singh in Washington, Rich McKay in Atlanta, and Ju-min Park in Seoul; Editing by David Gaffen, Sonali Paul, Ed Davies, Kim Coghill, Tomasz Janowski, Sharon Singleton and David Gregorio)