China placed a city of four million people under lockdown on Tuesday, ordering them not to leave home except in emergencies, in a bid to eradicate a COVID-19 cluster of just a few dozen confirmed cases.

Beijing imposed strict border controls after the coronavirus was first detected in China in late 2019, slowing the number of cases to a trickle and allowing the economy to bounce back.

But as the rest of the world opens up and tries to find ways to live with the virus, China has maintained a zero-COVID approach that has seen harsh local lockdowns imposed over handfuls of cases.

Tuesday's fresh restrictions came as China reported 29 new domestic infections -- including six in Lanzhou, the capital of Gansu province in the country's northwest.

The latest outbreak has been linked to the highly contagious Delta variant, with the tally hitting 198 cases since October 17.

Thirty-nine have been in Lanzhou.

Residents of the city will now be required to stay at home, authorities said in a statement, with the "entry and exit of residents" strictly controlled and limited to essential supplies or medical treatment.

Bus and taxi services had already been stopped in the city, and state media said Tuesday that Lanzhou station had suspended more than 70 trains, including on key routes to Beijing and Xi'an.

A Southern Airlines representative told AFP that all its flights from Beijing's Daxing airport to Lanzhou were cancelled due to public safety, with no resumption date given.

Health officials have warned that more infections may emerge as testing is ramped up in the coming days to fight the outbreak, which has been linked to a group of domestic tourists who travelled from Shanghai to several other provinces.

Northern outbreak

Strict stay-at-home orders have already been imposed on tens of thousands of people in northern China.

In Beijing -- which reported three new cases Tuesday -- access to tourist sites has been limited and the prominent Lama Temple was shuttered, while residents were advised not to leave the capital unless necessary.

About 23,000 residents in one housing compound in Changping district have been ordered to stay indoors after nine cases were found there in recent days, local outlet Beijing News reported.

Community mahjong and chess rooms have been closed, and residents have been told to reduce large gatherings.

Organisers on Sunday indefinitely postponed a marathon at which 30,000 runners were expected.

Mass testing is under way in 11 provinces and authorities have suspended many inter-provincial tour groups.

While the country's case numbers are extremely low compared with elsewhere in the world, authorities are determined to stamp out the latest outbreak with the 2022 Winter Olympics in Beijing just over 100 days away.

As part of China's strict enforcement of the zero-COVID policy, those deemed to have failed in controlling COVID-19 are often dismissed from their posts or punished.

On Tuesday, the official Xinhua news agency reported that the party secretary of Ejin Banner in the northern Inner Mongolia region had been sacked, "due to poor performance and implementation in epidemic prevention and control".

Hit by the latest wave, the city locked down about 35,000 residents from Monday.

Around 10,000 tourists were also placed under lockdown in Ejin, according to local media reports.

Six other officials were punished for their "slack response" to the latest flare-up, state media reported, and a local police bureau deputy director was removed from their position.

Beijing police have launched three criminal investigations into alleged COVID-19 safety breaches, deputy director of the city's public security bureau said Sunday.