The reclusive nation declared a state of emergency and leader Kim Jong-un ordered a total lockdown of the border city of Kaesong on 26 July after discovering a person with Covid-19 symptoms.
The Korean Central News Agency said on Thursday that 550,000 food and aid items have been shipped into Kaesong.
State media said the suspected case was a North Korean, identified in South Korea as a 24-year-old man surnamed Kim, who had earlier defected to South Korea before returning to Kaesong last month.
However, Yoon Tae-ho, a South Korean health ministry official, said: “The [defector] is neither registered as a Covid-19 patient nor classified as a person who came in contact with virus patients.”
North Korea’s public admission of its first potential case and the emergency response prompted outside speculation that it may be confronting a wider outbreak, despite its unwavering claims to being virus-free.
Suspicions have been raised further still by a report sent by the nation to the World Health Organisation that said it has quarantined 64 first contacts of the suspected Kaesong case and 3,571 secondary contacts in state-run facilities for 40 days.
Edwin Salvador, the WHO representative to North Korea, said Pyongyang informed WHO of the suspected first case, saying the person was tested for Covid-19 but the results were inconclusive.
All the country’s borders remain closed, group gatherings are banned, masks must be worn in public and all education settings are on an extended summer break, Dr Salvador told the Associated Press.
North Korea has also quarantined and released 25,905 people – 382 of them foreigners – since the end of December, according to Dr Salvador.
Foreign experts remain sceptical of North Korea’s claims that it has no cases, not least because of its long, porous border with China, where the virus emerged.
North Korea’s emergency steps suggest an outbreak there may have worsened, said Kim Sin-gon, a professor at Korea University College of Medicine in Seoul.
Hong Min, an analyst from Seoul’s Korea Institute for National Unification, said: “Though a really extensive local outbreak might not have occurred yet, it’s likely that a considerable number of people has been infected.”
Several other outside observers are all but certain the virus has already entered North Korea because it closed its border with China, its biggest trading partner, weeks after the world’s first known virus cases were recorded in China in December.
Monitoring groups in Seoul have also reported on virus cases and deaths in North Korea.
Additional reporting by agencies