Vietnam has closed the city of Da Nang to tourists after four new locally transmitted coronavirus were recorded, the country's first since April, reports BBC.
No tourists can enter the city for 14 days and extra flights are being laid on to fly out up to 80,000 visitors.
Vietnam has been lauded as a success story of the pandemic having acted early to close borders and enforce quarantine and contact tracing.
It has recorded just over 400 cases and no deaths.
But nearly 100 days after its last locally transmitted case, four new cases emerged in Da Nang, a central coastal city popular with domestic tourists.
Prime Minister Nguyen Xuan Phuc on Monday ordered Da Nang residents to re-implement social distancing and close all non-essential services.
He said the response had to be "decisive" but that he was not yet ordering a total lockdown of the city.
Links between cases unclear
The first new case - patient 416 - was a 57-year-old man who sought medical care on 20 July for flu symptoms.
He is now on a ventilator and, according to doctors quoted in local media, in a critical condition.
Officials say they do not yet know where he contracted the virus and that he had not recently left the city.
Contact tracing identified more than 100 people who had interacted with the man, but all returned negative tests.
However over the weekend, three more cases were confirmed, including one 17-year-old from neighbouring Quang Ngai province who had travelled home on a coach with people who had been at the Da Nang C Hospital.
Authorities have not yet commented on links between the four cases.
Da Nang C Hospital sealed its doors in response to the first diagnosis and Da Nang is reinforcing prevention measures.
Non-essential businesses have been ordered to close and gatherings of more than 30 people are banned. Tourism travel has been suspended for 14 days.
With international travel largely impossible, Da Nang had been promoted as a holiday destination for Vietnamese people.
Officials say up to 80,000 domestic tourists are in the city, so extra flights are being laid on take them home. People may be asked to quarantine on their return, according to media reports.
Hospitals across the country have also stepped up preventative measures, while the capital, Hanoi, has begun urging people to wear masks in public again.
Domestic football matches were also suspended on Sunday.
The new cases are a significant setback for Vietnam, which has been proud of its success in containing the virus.
It closed its borders to almost all travellers except returning citizens early on in the pandemic and requires anyone entering the country to quarantine in government facilities for 14 days and undergo testing.
Most of its cases have been detected in quarantine.
An extensive contact tracing and testing operation also meant it was able to quickly quash local outbreaks. It has so far recorded only 420 cases and no patients have died.
Its most famous patient, a British man who spent 68 days on a ventilator, was able to travel home earlier this month.