Xi'an, capital of Shaanxi province, is still facing a tough battle against the COVID-19 outbreak even though citywide testing is indicating a drop in infections, a health expert said on Thursday.

In the fourth round of nucleic acid testing of Xi'an's 13 million residents on Tuesday, 122 cases were confirmed, the local health commission said. This compared with 400 cases detected during testing on Sunday and Monday.

Another 155 confirmed locally transmitted cases were reported on Wednesday, taking the total number of cases in the city to 1,117.

"Relevant departments in Xi'an cannot let down their guard, despite a reduction in the number of positive results detected in the recent round of nucleic acid testing," said Zhang Boli, an academician at the Chinese Academy of Engineering. "When exactly that turning point comes also depends on the viral trend over the next few days," he told media.

Zhang forecast earlier that the Xi'an outbreak might reach a turning point in the second half of January and be under control before the Chinese Lunar New Year in February.

To help contain the spread of the coronavirus in the city, more than 20,000 police officers and auxiliary police are working day and night to ensure good social order and stability in local communities, a senior police officer said on Thursday.

"Now every community has at least one police officer on duty urging locals to strictly abide by relevant control measures and actively cooperate with relevant departments to fight against COVID-19," said Yin Hao, deputy director of the Xi'an public security bureau.

More than 2,100 traffic police have set up 133 checkpoints and expanded patrols to ensure that only vehicles engaged in pandemic control and prevention work and livelihood support use the roads, Yin said at a news conference on Thursday.

"The move aims to minimize the flow of people and vehicles engaged in pandemic prevention and control of the city," he said. "Xi'an's efforts to contain the outbreak are still complex, and the fight against COVID-19 has reached a critical period," said Yin.

Of the new patients, 83 are male and 72 are female. They include six children under age 3 and 16 people over 60, said Zhang Bo, deputy director of the health commission. "All have been sent to designated hospitals for treatment and are in stable condition," said Zhang. "The city government has organized health experts, senior doctors and medical staff to help."

Meanwhile, the city launched a new round of citywide nucleic acid testing at 10 am on Thursday. More than 9.06 million residents had samples collected as of 2 pm, he said.

Tian Gang, an official with the Xi'an market supervision and administration bureau, said the supplies of grains, oil, meat, vegetables, eggs, milk and other food staples are adequate and prices are stable.

"All the products sold in agricultural markets are required to have clearly marked prices, and the bureau and relevant departments have promised to take effective measures to fight any price gouging," he said.

Shanghai reported one asymptomatic COVID-19 case on Wednesday, the municipal government announced on Thursday. Authorities said the person was found to have come into close contact with an imported case that tested positive on Dec 23 during a seven-day self-monitoring period.