South Korea will send a chartered plane to Atlanta as early as Wednesday to bring back workers detained during a huge immigration raid last week on a car battery plant in the U.S. state of Georgia, a Korean Air spokesperson said on Tuesday.

A Korean Air Boeing 747-8i plane with 368 seats will fly from South Korea's Incheon to Atlanta, according to the spokesperson.

During the U.S. immigration raid, about 300 South Koreans were among 475 people arrested at the site of a $4.3 billion project by Hyundai Motor 005380.KS and LG Energy Solution 373220.KS to build batteries for electric cars.

South Korean Foreign Minister Cho Hyun is travelling to Washington to negotiate on issues such as seeking assurances that the detained Koreans will be allowed re-entry to the United States.

Addressing reporters before leaving for the U.S. on Monday, Cho called the detention of the South Koreans "a grave situation" and said he would work with Washington on measures to prevent similar incidents.

U.S. Department of Homeland Security Secretary Kristi Noem said many of the people detained in the Georgia operation would be deported.

"People that are in this country illegally need to know right now, today, that they have an opportunity to go home before they are detained," Noem told reporters on the sidelines of a ministerial meeting in London.

Details on how the workers may have breached immigration rules have not been released by authorities or the companies, but South Korean lawmakers said on Monday some may have overstepped the boundaries of a 90-day visa waiver programme or a B-1 temporary business visa.

Finance Minister Koo Yun-cheol said he had heard that some experts had travelled from South Korea to help with a test run of the factory, which was due to begin production in October.

"You need to get a visa to do a test run, but it's very difficult to get an official visa. Time was running out, and I think experts went to the United States," he said.

South Korean officials had kicked off the process of bringing the detained workers back home, a senior diplomatic official told reporters in Georgia after meeting with the workers in custody.

The raid was the largest single-site enforcement operation in the history of the Department of Homeland Security's investigative operations, and sent shockwaves through South Korea, a U.S. ally that has been trying to finalise a trade deal agreed with Washington in July.

Hyundai Motor is one of the biggest foreign investors in the United States and is among South Korean companies participating in a pledge of $150 billion in foreign direct investment in the U.S., which comes on top of a $350 billion fund that the South Korean government has separately pledged.

A spokesperson for the automaker said some staff had been asked to suspend non-essential trips to the United States.

The battery maker said last week it is cooperating with U.S. authorities and had paused construction work on the factory.

A Hyundai Motor spokesperson said last week none of the people detained were employed directly by the automaker and that production of EVs at the sprawling site was not affected.

The companies declined further comment on Monday.

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