SEOUL - South Korea is bracing for more imported cases of the coronavirus, as 16 infections on board a Russian-flagged cargo ship docked in the southern port city of Busan added to a recent spike in numbers coming through arrivals from Pakistan and Bangladesh.

Local cases also continued to grow, as health authorities warned a second wave of infections has already hit Seoul and greater Seoul - home to half of the country's 50 million people.

South Korea, a success story in beating back the virus in April, reported 46 new cases on Tuesday (June 23), 30 of them imported. This brings the total number of infections to 12,484, while the death toll has reached 281.

Half of the imported cases are linked to the cargo ship, which arrived from Vladivostok last Friday with a crew of 21. Three sailors were found with symptoms during a routine inspection by Korean authorities on Sunday.

Concern is rising over the possibility of community spread, as health authorities have identified 176 people who have come into contact with the infected crew.

They include port workers, repairmen and sailors from another Russia-flagged vessel, all of whom have been isolated and will undergo testing over the next few days.

The new cluster has raised questions over a possible lapse in virus screening procedures.

The Korea Centres for Disease Control and Prevention (KCDC) revealed in a briefing Tuesday that the Russian sailors received quarantine certificates from Busan port authorities even without submitting health questionnaires.

The ship's operator also did not inform South Korea their captain had returned to Russia a week ago after developing fever and other symptoms. His test results came back positive on Monday.

Quarantine officials said they will consider conducting physical inspections on board vessels arriving from Russia, instead of relying on electronic submissions of quarantine documents.

Separately, South Korea has suspended visas and flights for Pakistan and Bangladesh to curb a spike in imported cases from these two countries.

Of the 31 imported cases reported last Saturday (Jun 20), 16 were detected upon arrival from Pakistan and seven from Bangladesh.

Health officials said South Korea gets 3,000 to 4,000 international visitors a day, and border checkpoints are facing a shortage of workers to conduct the strenuous work of screening for the virus.

The number of imported cases rose to 90 last week - almost double the figure of the previous week.

Tuesday was the first time in almost 40 days that the number of imported cases (30) surpassed local infections (16). Daily imported cases fell to single digits after South Korea strengthened quarantine measures on international arrivals in April, but jumped to double digits in mid June.

Meanwhile, community infections continue to grow and pose a challenge to health authorities.

Seoul and greater Seoul accounted for most of the community infections this month, and authorities are considering tougher preventive measures.

The number of cases traced to a Seoul-based health products retailer, Richway, increased by four to 202 on Tuesday, while a day care centre for seniors, also in Seoul, registered two more infections for a total of 45.

Seoul mayor Park Won-soon cited expert warnings that the number of daily cases could grow to 800 in a month's time, given the high R-number of 1.79. The R-number, or reproduction rate, refers to the average number of people infected by a confirmed patient.

The looming crisis has come much sooner than earlier forecasts of a second peak in autumn, he said, urging people to follow preventive measures such as wearing masks and avoiding confined spaces.

President Moon Jae-in has also stressed the importance of guarding against further spread of the virus, noting "it is no exaggeration to say that the stability of the coronavirus situation depends on the capital area".

Speaking at a Cabinet meeting on Tuesday, he also urged Parliament to approve a third supplementary budget worth 35.3 trillion won (S$40 billion) submitted three weeks ago, as a further delay would "increase the suffering of the people" and derail plans for economic recovery.

In an expansion of preventive measures implemented since June 10, certain businesses are now required by law to use a quick response (QR) code system to record visitor entries to help facilitate contact tracing. The system applies to high risk venues such as buffet restaurants, private academies and logistics facilities.