Hundreds of South Korean workers were headed back to Seoul after they were netted in a huge US immigration raid at a battery plant / AFP

Hundreds of South Korean workers were headed back to Seoul on Friday after their detention in a US immigration raid that Hyundai warned will delay completion of its battery factory. 

South Korean workers accounted for most of the 475 people arrested last week at the Hyundai-LG battery plant under construction in Georgia, prompting tense negotiations between Seoul and Washington, staunch security allies.

A specially chartered Korean Air Boeing 747-8I carrying 316 South Koreans and 14 foreign employees departed Atlanta's Hartsfield-Jackson International Airport on Thursday, Seoul's foreign ministry said.

"Everything at Atlanta went smoothly," a foreign ministry official told AFP on Friday, ahead of the workers' expected arrival at 2:00 pm (0500 GMT).

"The plane departed as scheduled with the planned number of passengers."

The Georgia raid was the largest single-site operation conducted since US President Donald Trump launched a sweeping immigration crackdown, a top political priority since he returned to office in January.

Experts say most of the detained South Korean workers were likely to have been on visas that do not permit hands-on construction work.

President Lee Jae Myung called the raid "bewildering" and noted it could have a chilling effect on future investment. 

He added that Seoul was negotiating with Washington "to ensure that visa issuance for investment-related purposes operates normally". 

Asia's fourth-largest economy maintains multiple plants in the United States, and has heeded Washington's push to onshore manufacturing and boost investment in America. 

At the Hyundai factory site, construction will now be set back at least a few months due to labour shortages, Chief Executive Officer Jose Munoz said.

"This is going to give us minimum two to three months delay, because now all these people want to get back," he said.

"Then you need to see how can you fill those positions. And, for the most part, those people are not in the US."

- Minimise impact -

LG Energy Solution -- which said 47 of its employees were arrested, along with about 250 people working for its contractor -- thanked the Seoul government for its support.

Seoul sent a task force and flew in top officials to negotiate, with a focus on ensuring that workers would not suffer future repercussions should they seek to re-enter the United States.

"We are especially grateful for their exceptional efforts... for their meticulous attention to addressing various concerns, including ensuring no disadvantages upon re-entry," the firm said in a statement sent to AFP.

Images of the workers being chained and handcuffed during the raid caused widespread alarm in South Korea, and Seoul said the government had negotiated to make sure the workers were not handcuffed again as they were repatriated.

The raid came less than a month after Trump welcomed Lee to the White House.

The site of the raid is a $4.3 billion venture to build a battery cell manufacturing facility in Georgia.

Many South Korean companies bring their own workforce during project development periods, with industry sources telling AFP it is common practice to use visa workarounds to avoid project delays.

LG said it remained committed to its US projects, adding that it was also working to minimise "any business impact resulting from this incident".

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