US Immigration and Customs Enforcement agents raiding the Hyundai-LG plant in Georgia on September 4 / US Immigration and Customs Enforcement / AFP

A chartered plane left Seoul for the United States Wednesday to repatriate hundreds of South Korean workers detained in an immigration raid, flag carrier Korean Air told AFP.

South Koreans made up the majority of 475 people arrested at a Hyundai-LG battery plant under construction in the US state of Georgia last week, according to immigration agents. 

The operation was the largest single-site raid conducted under US President Donald Trump's immigration crackdown, according to an investigating agent. 

A Korean Air Boeing 747-8I, which seats over 350 passengers, left Seoul Wednesday, a company representative told AFP.

"A timeline for the return flight has not been finalised," she added.

Citing diplomatic sources, Yonhap news agency reported that the plane would leave the United States with the workers at 3:30 am Thursday (1830 GMT Wednesday).

Seoul's Foreign Minister Cho Hyun, who is now in Washington for talks on the issue, has called the mass detention of South Koreans a "grave situation" and pledging to secure the workers' swift return "in good health".

Before departing, Cho told South Korean MPs that "a tentative agreement" had been reached with US authorities to ensure the detained workers would not face penalties, such as a five-year ban on re-entry.

"I can tell you that negotiations are going well," he said.

- 'Great responsibility' -

The South Korean government is reportedly seeking to have the detained workers leave the United States under voluntary departure rather than deportation. 

Minister Cho is expected to raise the issue during his meeting with US Secretary of State Marco Rubio in Washington on Thursday morning.

"I feel a great responsibility that our citizens have been arrested in this manner," Cho said during a meeting with South Korean business leaders in Washington on Tuesday. 

Diplomatic officials had been dispatched to Georgia to mount a "full-front response", he added, with the consulate general in Atlanta in contact with local authorities.

In addition to being a key US security ally, South Korea is Asia's fourth-biggest economy and a major automaker and electronics producer, and its companies have multiple plants in the United States.

Seoul also heeded Washington's repeated call during tariff negotiations for global investment in the United States.

The site of the raid is a $4.3 billion joint venture between two South Korean firms –- Hyundai and LG Energy Solution –- to build a battery cell manufacturing facility in Georgia.

Experts said most of the detained South Korean workers were likely to hold visas that do not allow for hands-on construction work.

kjk/ceb/fox