China reported its highest daily rise in local Covid-19 cases in 21 months as infections more than doubled in the northwestern city of Xian, China's latest hot spot.

The city of 13 million, which entered its fourth day of lockdown, detected 155 domestically transmitted cases with confirmed symptoms for Saturday, up from 75 a day earlier, official data showed on Sunday.

That drove the national daily count to 158, the highest since China managed to contain a nationwide outbreak in early 2020.

Xian, with 485 local symptomatic cases reported for the Dec 9-25 period, has imposed heavy-handed measures to rein in the outbreak, in line with Beijing's policy that any flare-up should be contained as soon as possible.