South Korea’s latest infection cluster is linked to a Russia-flagged cargo ship where at least 16 out of the 21 sailors tested positive for the novel coronavirus that causes COVID-19, raising questions over screening loopholes at the country’s seaports.

Busan officials said in a briefing Tuesday that they conducted coronavirus tests on the sailors at 1:30 p.m. Monday upon receiving notice that the ship’s captain, who returned to Russia after experiencing fever and other symptoms, was diagnosed there earlier that morning. The ship, which departed from Russia’s Vladivostok, docked in the port city at around 8 a.m. Friday.

So far, 176 people -- including customs officials, ship repair workers and port office staffers -- have been identified as contacts of the infected crew members and quarantined accordingly. All of them are set to undergo testing Wednesday.

Following investigations revealed another cargo ship, also of Russian origin, that is berthed nearby belonged to the same company. Tests carried out Tuesday yielded a positive result for one of the 21 crew members. Some 90 people who came into contact have been quarantined.

The sailors are receiving treatment at the Busan Medical Center.

“At this point, it’s hard not to assume that the cargo ship outbreak will grow further,” Korea Centers for Disease Control and Prevention’s Deputy Director Kwon Jun-wook said in a briefing Tuesday.

Officials at the KCDC said the Busan port authorities had not taken health questionnaires from the Russian sailors before allowing them to dock and disembark, pointing to a possible negligence in the process.

Coupled with a recent uptick in imported cases, the newly detected cluster has prompted health authorities to introduce amplified border checks to better screen potential infections coming in from abroad.

The number of new coronavirus cases in Korea bounced back Tuesday, recording 46 more cases, 30 of which were imported.

The chief of the municipal office’s health department, Ahn Byoung-seon, said finding a place to house the increasing number of people subject to quarantine -- the vast majority of them tied to overseas travels -- and ensuring they were following through with the order were adding to its administrative burden.

“Some housings are not apt for proper isolation, and the city is arranging hotels and other facilities to accommodate (the quarantined individuals),” she said.

Two patients were confirmed Tuesday in Busan, one of whom arrived recently from Kazakhstan and the other from Pakistan. There are currently 2,342 international travelers and their contacts placed under quarantine in the city.

Senior health official Yoon Tae-ho told reporters Tuesday that all those arriving here are obliged to report any symptoms to authorities.

He added that the offices in charge of coronavirus screening at border checkpoints, mainly at airports, were facing workforce shortages amid a steady influx of travelers.

“About 3,000 to 4,000 air passengers come here from outside the country each day. Checking all of them for presence of symptoms and testing those who display symptoms are strenuous jobs,” he said, adding that more workers will be recruited in the fall or winter to hopefully relieve the workload.

The number of new cases associated with overseas travel reached 90 last week, almost double the figure from the week prior. Starting Tuesday, visas are temporarily suspended for those from Bangladesh and Pakistan, where a high proportion of arrivals have been found to be infected. Nonessential flights between Korea and the two countries have been canceled as well.

Also coming into effect Tuesday is a regulation that mandates the use of a QR code-based ID system at private cram schools, buffet-style restaurants, logistics facilities and door-to-door sales businesses -- all of which have been sites of local outbreaks. This is an expansion of measures implemented June 10 intended to facilitate contact tracing at risky venues.

One more person died in the 24 hours ending Monday at midnight, bringing the death toll to 281. Total nationwide infections topped 12,484.

By Kim Arin (arin@heraldcorp.com)