Vietnam recorded its first coronavirus outbreak in almost two months on Thursday, with more than 80 new cases reported as authorities began testing tens of thousands of people to contain the spread.
The infections were detected in northern Vietnam, where one cluster has been linked to a case of the more contagious coronavirus variant first spotted in Britain.
The figure was a new daily record for the communist nation of 96 million people, which has been widely praised for its handling of the pandemic -- recording just over 1,500 cases before the new cluster and only 35 deaths.
The latest outbreak in the provinces of Hai Duong and Quang Ninh is "more complicated and serious" than anything Vietnam has dealt with before, Deputy Prime Minister Vu Duc Dam said.
"We aim to isolate and fight the outbreak to prevent its spread as quickly as possible."
In Hai Duong province, the cases are thought to be linked to a factory worker who tested positive for the virus when she landed in the Japanese city of Osaka. She was found to have the British variant.
The city where the factory is located has been closed off and 220,000 people are in lockdown, while medical workers are rushing to test tens of thousands of people in the area.
In Quang Ninh, on the border with China, one of those infected is a security official at Van Don airport, where international passengers have been arriving before being sent into mandatory quarantine.
The airport has been shut while passenger vehicles are banned from leaving or entering the province.
The outbreak comes at a sensitive time for Vietnam, with its twice-in-a-decade Communist Party congress beginning on Tuesday to select new leadership.
A long Lunar New Year break will follow in February, when millions of people are expected to travel to see their families.
Vietnam last recorded a community transmission case of the virus in December, and had kept infections low thanks to mass quarantines, extensive contact-tracing and strict controls on movement.
Most inbound commercial flights have been suspended since March but Vietnamese citizens stuck abroad and some foreigners have been allowed to enter.