BUSAN -- More than 50 patients and workers at a nursing hospital in Busan have been diagnosed with COVID-19, municipal authorities said Wednesday, in the southern port city's latest coronavirus cluster.

The authorities said 43 patients of the Busan nursing hospital, located in the city's Mandeok neighborhood, and its 10 staff members tested positive for the new coronavirus. One of the infected patients has already died, they added.

The mass virus infections have come to light after an assistant nurse in her 50s was confirmed to be infected Tuesday, prompting the authorities to conduct the coronavirus tests for all 261 patients and staff.

The hospital, named Haeddeurak, was immediately put under cohort isolation, they said.

The outbreak of mass coronavirus infections came days after the South Korean government relaxed its social distancing measures from Level 2 to Level 1. Busan, South Korea's second-largest city with a population of 3.4 million, has so far reported 486 coronavirus cases and four deaths from the disease.

The authorities said the Haeddeurak Nursing Hospital is the largest cluster infection ever witnessed by Busan. It is the first time in the city that more than 50 infections have been traced to a single location.

They are now carrying out an epidemiological survey to determine how and where the assistant nurse contracted the virus.

The nurse reportedly told health officials that she developed a fever last Thursday after coming in contact with the deceased patient. Her temperature rose to 38 C at that time. She then visited a COVID-19 screening clinic Monday after experiencing muscle pain.

They also fear that more infections could be identified from among other hospital patients, mostly aged people.

In recent weeks, a dozen coronavirus cases have been reported from many locations of the Mandeok neighborhood, including restaurants, bath houses and swimming pools, prompting the municipal government to close 18 public parks in the district on Oct. 1 and advise all commercial establishments to thoroughly follow quarantine guidelines.

The government is also planning to administer coronavirus tests to about 1,400 patients and staff members at all 11 nursing hospitals and homes in Mandeok. (Yonhap)