South Koreans made up the majority of 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 from Seoul today, a company representative told AFP.

“The chartered plane left around 10.20am for the US,” said the spokeswoman.

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

Local media broadcast footage showing what they described as the chartered plane taking off from Incheon International Airport.

Citing diplomatic sources, Yonhap news agency reported that the plane would leave the US with the workers at 6.30pm today.

Seoul’s foreign minister Cho Hyun headed to Washington on Monday for further talks, calling 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.

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 US.

Seoul has also heeded Washington’s repeated call during tariff negotiations for global investment in the US.

The site of the raid is a US$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.