Beijing,  Even as China continues to pursue a 'zero-Covid' strategy, the northwestern city of Xi'an with 13 million, has entered the fifth day of strict lockdown.

To curb the Covid outbreak, officials on Sunday conducted a mass disinfection programme in Xi'an with roads, buildings and even the air in open spaces disinfected, South China Morning Post reported.

The residents were given just a few hours' notice on Sunday afternoon that a "full-scale" deep cleaning would start at 6 p.m. to "further improve the prevention and control of the epidemic".

Residents were also asked to close their doors and windows, not to bring in any laundry from open balconies, avoid touching building exteriors, as well as plants and trees, and to wash their hands if they did come into contact with any external surfaces.

However, it is unclear what chemicals were used in the disinfection operation, the report said.

It was followed by a fourth round of mass testing and further restrictions.

Since Xi'an's first local Covid-19 case was reported on December 9, daily case numbers have been growing and expanding to more communities, to reach a total of 635 cases by Sunday.

However, according to health officials, the situation remained "dire and complex" and they expected the number of local infections to remain high in coming days, the report said.

While lockdowns and mass testing would help bring the situation under control, a mass disinfection campaign was likely to be "of little use", virologist Jin Dong-Yan, a Professor at the University of Hong Kong, was quoted as saying.

According to Jin, authorities were grappling with a situation where it appeared contact tracing had "fallen far, far behind" transmission, raising the challenge of containing the situation.

"They overdo everything, but they are worried because they don't know the source of (some of) the active transmission... there are indications that some of the cases are not linked or have no contact, so that's why they worry that the virus might be in the air, but that should not be true," he said of the disinfection programme.

"There might be airborne transmission, but that would be under very specific circumstances." While it is thought to be possible to pick up an infection from a contaminated surface, this would not be driving most transmission, he added.