Xi'an, capital of northwest China's Shaanxi Province, introduced a slew of restrictive measures to curb a resurgence of COVID-19 cases that have spread to other provinces across China.

All communities and companies are now under closed-off management, local authorities said at a press conference on Wednesday, as positive cases have been detected outside the city's restricted areas.

The city has one high-risk COVID-19 area and 40 medium-risk areas.

Genetic sequencing of the confirmed cases showed this round of outbreak was caused by the Delta strain and had led to community spread to a "certain scale," authorities said.

Non-essential places in Xi'an are temporarily halted from conducting business, and citizens are suggest not to leave the city.

Large-scale meetings, events, forums and training are discouraged, and work from home is advised, according to the authorities.

Xi'an has accounted for 143 of the 149 local transmissions detected in Shaanxi Province since December 9, and registered 52 of the 53 local infections in the province on Tuesday.