Vietnam is evacuating 80,000 people from the central tourism hot spot of Danang after three residents tested positive for the coronavirus over the weekend.
The evacuation will take at least four days with domestic airlines operating approximately 100 flights daily from Danang to 11 Vietnamese cities, the government said in a statement.
Those returning from Danang to other parts of the country would be required to quarantine at home for 14 days, the health ministry said.
Following the discovery of the new cases, Prime Minister Nguyen Xuan Phuc ordered police to step up a crackdown on illegal immigration to the country.
State media on Sunday said police in Danang had arrested a 42-year-old Chinese national it said was the head of a criminal group that helps people illegally enter Vietnam from China.
Authorities have not officially linked the new cases in Danang to illegal immigration.
However, in a separate statement on Monday, the government said authorities in Ha Giang province, which borders China, had caught more than 1,500 people illegally crossing into the province since May.
So far, the country had kept its total tally of reported infections to just 420, with no deaths.