Beijing has largely succeeded in controlling the spread of the coronavirus within its borders through travel restrictions and snap lockdowns, but frequent domestic flare-ups have tested its no-tolerance strategy in recent months.

The three positive cases are friends who travelled to the nearby city of Suzhou together last week, Shanghai health authorities said at a press conference Thursday evening -- adding that all had been fully vaccinated.

Over 500 flights from Shanghai's two major airports were cancelled on Friday, data from flight tracker VariFlight showed.

China reports 17 new coronavirus cases for Nov 20 vs 23 day earlier

The Shanghai government also instructed that all cross-provincial package tours involving the city -- a major commercial and tourism hub -- would be cancelled.

Six Shanghai hospitals have also suspended outpatient services.

"China has accumulated lots of experience in 'dynamic zero-Covid', so our strategy won't change," said Zhang Wenhong, head of the Shanghai Covid prevention expert task force, at a Thursday briefing.

He warned that this could become a "normalised anti-epidemic situation that we may encounter again" in the future.

Around 100 kilometres away, Suzhou -- which has a population of some 13 million people --- closed down tourist attractions and required residents to provide negative test results to leave the city.

Authorities in Beijing are on high alert for any potential outbreaks in the lead up to February's Winter Olympics, which will see an influx of foreign athletes, media and officials.

All schools were closed in the small satellite city of Xuzhou, which also stopped its two million residents from heading out of the city on bus services, after a close contact of one Shanghai patient was found there.

A university campus in neighbouring Hangzhou was put under lockdown after a staff member was discovered to be a close contact of a confirmed case, state media reported.

With the leadership determined to host a Covid-free Olympic Games, Beijing's Olympic Park has already been sealed off as part of the event's "closed-loop" bubble, state media reported.