A strike by South Korean truckers poses an early test for President Yoon Suk-yeol after a month in office, with the dispute threatening to distract from his agenda while raising the risk of long-term antagonism with powerful trade unions.

Thousands of truckers went on strike for a fourth day on Friday in a protest over pay as fuel costs surge, disrupting industrial production, slowing port operations and posing new risks to a strained global supply chain.

A political novice and former prosecutor-general, the conservative Yoon criticised trade unions he sees as hardline during the campaign for his May election victory, calling them "front-line guards" for his political opponents and instigators of trouble despite the generous salaries their members earn.

Yoon has kept his distance from this week's strike, saying it was up to the unions and management to solve it, but Shin Yul, a political science professor at Myongji University in Seoul, said a prolonged strike could undermine support for Yoon and distract him from his economic and security agenda.

"Labour strikes tend to hurt the popularity of any administration," Shin said.

Yoon's focus is on tackling surging inflation at home and working with the United States and other allies on responding to North Korea's intensifying weapons tests, amid expectations it is about to conduct its seventh nuclear test.

Try as Yoon might to steer clear of the industrial dispute, it has a political element at its core that he will find hard to ignore the longer the strike goes on.

The truckers, largely self-employed, are demanding pay increases and an extension of a "safe rates" system, introduced during the COVID-19 pandemic to guarantee minimum freight rates, which is due to expire in December.

Legislation to extend the system was introduced in parliament last year, under the administration of a liberal president who enjoyed the backing of the trade unions.

Now the bill is bogged down in the legislature with no sign of a compromise.

'DILEMMA'

The liberal Democrats, who hold a majority in parliament, accuse Yoon's conservative party of blocking the bill. Yoon's supporters deny that, accusing the Democrats of dodging their responsibility for the deadlock.

South Korea has a long record of activist trade unions taking on governments, especially conservative ones.

"It poses a thorny dilemma," Kim Dong-won, a management professor at Korea University's business school, said of Yoon's options. "If he decides to crack down on the strikers, it might lead to criticism of labour repression and the sort of severe conflicts that past conservative leaderships have faced."

"But the new government won't want to look too soft and let this become a source of future, potentially more intensive, labour rows," Kim said.

Critics of the striking truckers say giving in to them would put a big burden on industry just as the country is grappling with inflation at near 14-year high and after the previous liberal government raised the minimum wage by a record amount.

Yoon is trying to remain aloof, calling for the union and management to resolve the dispute on the basis of law and principles.

"If the government intervenes all the time and gets too involved according to public sentiment, it doesn't help either side at all to build their own capability and environment to smoothly resolve the problem," he told reporters this week.

Yoon brushed off his campaign-trail criticism of the unions.

"How can a person who is hostile toward labour become a politician?" he said.