Sailors aboard ships coming from Russia will be required to submit a certificate showing they have tested negative for the new coronavirus for entry into South Korea as cluster infections traced to Russian ships docked here have continued to swell, health authorities said Wednesday.

Starting on Aug. 3, seafarers on all vessels departing from Russia should turn in such certificates, according to South Korean health authorities. The documents should be issued within 48 hours of departure.

In order to stem further inflows of cases from overseas, South Korea has designated six nations -- Bangladesh, Pakistan, Kazakhstan, Kyrgyzstan, the Philippines and Uzbekistan -- as "high-risk" countries for beefed-up quarantine measures.

Arrivals from the six high-risk countries are required to submit a certificate showing they tested negative for the coronavirus.

Infections traced to Russia ships have become a new source of cluster infections here since last month, with 19 virus cases tied to two Russia-flagged vessels docked in the southeastern port city of Busan reported first. Infections tied to Russian ships have reached 91 in total so far.

More Russian seafarers were confirmed to have been infected with the new coronavirus on Wednesday.

According to health authorities, 12 more cases were reported from a Russian ship docked in Busan, raising the total caseload tied to the ship to 44.

Last week, 32 Russian sailors on the ship carrying 94 crew members were confirmed to have contracted COVID-19.

In addition, one Russian sailor aboard a cargo ship docked in Incheon, west of Seoul, also tested positive for the virus.

Carrying 20 crew members, the 6,800-ton cargo vessel departed from Vladivostok on July 21 and entered the Incheon port on Sunday. Nineteen other sailors tested negative for the virus.

Since late June, South Korea has been conducting on-board quarantine checks of Russian ships arriving in Busan ports.

In the second half, around 38,000 ships, including container cargo vessels, are expected to arrive in South Korea, the government said.

"To reduce a blind spot in the virus fight at ports, we've come up with quarantine measures that include ways to minimize contacts with foreign sailors in case of ship repair work," Son Young-rae, a senior health official, said in a briefing. (Yonhap)