SEOUL (AFP, REUTERS) - South Korea on Thursday (May 28) re-imposed a series of coronavirus social distancing measures it had eased early this month, as a series of clusters that saw the country registering its highest number of daily cases since early April threatened to challenge its success in containing the epidemic.

The country has been held up as a global model in how to curb the virus, but reported its biggest spike in new infections in nearly two months as life appeared returning to normal.

The new cases - mostly centred in the Seoul metropolitan area, where half the South Korean population lives - prompted officials to strengthen social distancing rules that were eased on May 6.

Museums, parks and art galleries will all be closed again from Friday for two weeks, said Health Minister Park Neung-hoo, while companies were urged to re-introduce flexible working, among other measures.

"We have decided to strengthen all quarantine measures in the metropolitan area for two weeks from tomorrow to June 14," he said.

Citizens were also advised to refrain from social gatherings or going to crowded places - including restaurants and bars - while religious facilities were asked to be extra vigilant with quarantine measures.

There were no new delays, however, to the phased re-opening of schools that is currently underway.

"The next two weeks are crucial to prevent the spread of the infection in the metropolitan area," Mr Park said, adding: "We will have to return to social distancing if we fail."

The Health Minister said the government will be forced to re-impose an all-out social distancing campaign if the country sees more than 50 new cases for at least seven consecutive days.

Officials announced 79 new cases on Thursday - taking its total to 11,344 - the largest increase since 81 cases were announced on April 5.

An outbreak at a warehouse of e-commerce firm Coupang in Bucheon, west of Seoul, has seen 69 cases, said the Korea Centres for Disease Control and Prevention.

Around 4,100 workers and visitors to the building were under self-isolation, with more than 80 per cent tested so far, Vice-Health Minister Kim Gang-lip told reporters.

"We are expecting the number of new cases linked to the warehouse to continue rising until today as we wrap up related tests," he added.

The country endured one of the worst early outbreaks of the disease outside mainland China, and while it never imposed a compulsory lockdown, strict social distancing had been widely observed since March.

But it appears to have brought its epidemic under control thanks to an extensive "trace, test and treat" programme. South Korea's robust programme of testing earlier this year was credited with helping the number of deaths comparatively low in a global pandemic that has now killed more than 350,00.

The warehouse cluster appears linked to an outbreak that emerged in several Seoul nightclubs and bars in early May, the KCDC said.

Unlike many countries, South Korea didn't impose a strict lockdown to counter the new coronavirus, but officials said if new cases keep rising, they may consider issuing new guidelines.

Health officials said on Thursday they would be conducting on-site inspections of logistics centres across the country, to develop better policies for preventing outbreaks at such facilities.

Coupang, backed by Japanese tech conglomerate SoftBank Group, said it closed the Bucheon facility on Monday.

It said on Thursday it had also closed a separate facility in Goyang, in the Seoul suburbs, after an employee tested positive there.

"As soon as the employee's diagnosis was confirmed, Coupang sent home and self-quarantined employees who had contact with the employee," the company said in a statement.

The spreading outbreak and warehouse closures come as Coupang and other e-commerce firms scramble to keep up with a surge in orders as more people opted to shop from home during the coronavirus outbreak, despite the absence of a strict lockdown.

In February, March and April, sales of South Korean online retailers including Coupang jumped 34 per cent, 17 per cent and again 17 per cent respectively from the same months a year ago, according to trade ministry data.

That compared with offline retailers' sales that dropped 7.5 per cent, 18 per cent and 5.5 per cent in the same three months from a year earlier.