Its Center for Disease Control (CDC) said everyone who returned to the city from areas that have since seen outbreaks must report to authorities for testing.

They should also isolate themselves as they wait for their test results, it added.

The city has found five out of its six Covid cases in Vietnam's latest wave within a mere four days, raising the specter of an outbreak.

More than 244,700 people have entered the city since the holidays that lasted four days from April 30 to May 3, of whom 5,100 have been tested. All have tested negative so far.

The city has vaccinated over 64,200 people, mainly medical workers, airport employees and people working in quarantine zones.

The six cases so far include a woman and her two children in District 3 and two coworkers in a company in the same district.

Contact tracing efforts are ongoing to find their sources of infection.

More than three weeks into the latest wave, Vietnam has had 1,786 cases in 30 cities and provinces, with four people losing their lives.