BUSAN -- More seafarers on a Russia-flagged ship docked in the southern port city of Busan were confirmed to have been infected with the new coronavirus, health authorities said Tuesday.

Four additional Indonesian sailors on the fishing vessel have tested positive for COVID-19.

Health authorities believe they might have contracted the virus through the captain of the ship, who tested positive for the novel coronavirus last week.

So far, 10 virus cases have been traced to the Russia-flagged ship, with seven being its seafarers, according to the Korea Centers for Disease Control and Prevention.

Since June, health authorities have detected more than 90 virus cases at Russian ships docked in Busan.

Since Aug. 3, South Korea has required crewmen of ships departing from Russia to submit a certificate showing they tested negative for the new coronavirus upon arriving in South Korea. The certificates must be issued within 48 hours prior to the departure.

Health authorities have also been conducting onboard quarantine checks of Russian ships arriving in Busan ports since late June. (Yonhap)