Vietnam reintroduced social distancing measures in the central city of Danang on Sunday after the country reported four locally transmitted coronavirus cases over the past two days, the first in more than three months.
The Southeast Asian country was back on high alert after the government on Saturday confirmed its first community infection since April, and another case early on Sunday, both in the tourism hot spot of Danang.
The two new cases included a 17-year-old boy in Quang Ngai province and a 71-year-old woman in Danang, the government said late on Sunday, bringing the total number of reported cases in the country to 420.
Authorities gave no further detail on how the new infections were contracted nor whether they were believed to be linked.
The city's social distancing measures would come into force until further notice, the government said on its website.
Danang will stop receiving inbound tourists for 14 days and all religious, sports and cultural events will be suspended.
Wearing masks in public places in the city is now compulsory and gatherings of more than 30 people at public places are banned, the government said.
The detection of the new cases, which come as Vietnam seeks to revive its economy and resume international commercial flights, has prompted authorities to step up a crackdown on illegal immigrants.
State media said dozens of migrants from China have been detected in Danang over recent days.
Thanks to strict quarantine measures and an aggressive and widespread testing programme, Vietnam has kept its virus total to an impressively low level with no deaths.