North Korea's state media has reported four new fever cases, saying they were suspected of being infected with the "malignant epidemic" - just two weeks after the country declared victory over Covid-19.
The isolated nation, which has maintained a rigid blockade since the start of the pandemic, confirmed an Omicron outbreak in the capital Pyongyang in May.
North Korean leader Kim Jong-un - who also fell ill during the outbreak - declared victory over the virus earlier this month and ordered the lifting of the country's "maximum emergency epidemic prevention system" as officially reported cases fell to zero.
But four new fever cases were recorded in Ryanggang province - which borders China - on 23 August, the official Korean Central News Agency (KCNA) said, citing data from the state emergency epidemic prevention headquarters.
Authorities "immediately locked down the area where fever cases occurred and at the same time urgently mobilised rapid mobile anti-epidemic teams and rapid diagnosis and treatment team," KCNA said.
They are also "taking measures to probe into the cause of the fever outbreak," it said.
North Korea refers to "fever patients" rather than "Covid patients" in case reports, apparently due to a lack of testing capacity.
It has recorded nearly 4.8 million "fever" infections and just 74 deaths for an official fatality rate of 0.002%, according to state media. It has reported no new cases since July 29, until now.
Experts, including the World Health Organization, have long questioned Pyongyang's Covid statistics and claims to have brought the outbreak under control.
North Korea has one of the world's worst healthcare systems, with poorly equipped hospitals, few intensive care units and no Covid treatment drugs, experts say.
It is not believed to have vaccinated any of its 25 million residents, although it may have received some vaccines from China, Seoul-based specialist site NK News has reported.
Pyongyang has blamed the outbreak on the South Korea and warned of "retaliation".