A "wartime state" has been declared in Urumqi, the capital of China's western Xinjiang region, after a spike in cases of coronavirus.
Officials on Saturday said 17 new cases had been recorded and strict measures on movement had been imposed.
Although the figure appears low, China has recorded very few significant outbreaks since Covid-19 emerged in the city of Wuhan late last year.
China is now not in the top 20 in terms of either infections or deaths.
It has recorded just over 85,000 infections and 4,600 fatalities, according to Johns Hopkins university research.
What is happening in Urumqi?
The capital of the Xinjiang Autonomous Region has a population of about 3.5 million.
The latest infections began to be detected on Wednesday, the first in the region for months, prompting the cancellation of almost all flights in and out of the city. Subway services were also suspended.
On Saturday, officials said the total of cases was 17. One official told a state briefing: "The whole city has entered a 'wartime state', and will suspend all kinds of group activities."
The latest lockdown measures include:
- Mass screening in buildings where new cases have been detected, later expanding to the whole of Urumqi
- Visits to other households are restricted, as are large gatherings
- Residents urged not to leave the city unless absolutely necessary and must be tested if they do
Rui Baoling, the director of the disease control and prevention, said the main cluster had been detected in the Tianshan district of Urumqi but although the "epidemic has developed rapidly", she added that the "situation is generally controllable".
Xinjiang has this week reported another 23 cases listed as "asymptomatic", with 269 people under "medical observation".
How has China coped with outbreaks?
The vast majority of coronavirus deaths and most of the infections in China occurred in Wuhan at the start of the year.
Since then, the only large-scale cluster was reported in June in Beijing, where more than 330 people were infected before the outbreak was contained.
However, some have questioned China's reporting of numbers on the outbreak.
China has certainly taken tough measures to prevent any outbreak expanding, imposing strict controls where necessary, including shutting transport and introducing mass testing.
For Friday, only 22 new cases were reported for the whole of mainland China, and that includes 16 of Urumqi's cases.
To put this in perspective, Florida on Saturday registered a rise of more than 10,000 cases.
Why Xinjiang is also in the news
Xinjiang is home to the mainly Muslim Uighur people and other Turkic minorities and is subject to intense state control.
The BBC's China correspondent Stephen McDonell says there is the added sensitivity of controversial detention camps where hundreds of thousands of mostly ethnic Uighurs have been interned as part of a mass "de-radicalisation" programme.
He says if the coronavirus got into one of those camps the same way it has in prisons, the possibility of widespread infection would be considerable.
Xinjiang is one of a number of issues that have contributed to worsening US-China relations.
Last week, the Trump administration announced sanctions against Chinese politicians who it says are responsible for human rights violations against Muslim minorities in Xinjiang.
China rejects the accusations. On Thursday, foreign ministry spokesperson Hua Chunying invited US Secretary of State Mike Pompeo to visit Xinjiang to see for himself that there were no such violations.