Xi'an, capital of Shaanxi province, has tightened control measures to contain the spread of the novel coronavirus after the number of confirmed COVID-19 cases in the city continued to witness sharp growth.

No vehicles, except those engaged in pandemic control and livelihood support, were allowed on the roads since Monday.

Zhang Bo, deputy director of Xi'an city health commission, said the metropolis reported 150 new confirmed COVID-19 cases on Sunday, pushing the city's total number of coronavirus patients to 635 by the end of Sunday.

It was the second day that Xi'an reported more than 150 confirmed cases in a single day.

The new reported cases－78 men and 72 women－included two children aged under three and 12 seniors over 60, Zhang said at a news conference on Monday.

All residents are required to stay at home unless they have to go out for nucleic acid testing, according to a notice issued by the city's public security bureau. The previous policy that allowed one person from a family to buy daily necessities every two days was suspended until further notice.

Those who violate the COVID-19 policies will be punished, with a maximum of 10 days in detention and a fine of 500 yuan ($78), the notice said.

Zhang said Xi'an added 35 medium risk areas on Sunday, and the city now has 150 medium-risk areas, plus a high-risk one.

To detect those infected, the city has started a new round of mass nucleic acid testing beginning on Monday.

More than 31,000 medical staff have been sent to 5,077 sample collecting centers citywide to help identify possible patients and asymptomatic carriers, he said.

As of 4 pm Monday, 6.42 million residents have had their samples collected.

The big increase of COVID-19 patients in Xi'an has brought the total number of confirmed cases in Shaanxi province to 651 by the end of Sunday.

Ma Guanghui, deputy director of Shaanxi Provincial Health Commission, said Shaanxi province reported 598 confirmed cases in the previous week, with the youngest only six months old and the oldest over 90, he said.

Gene sequencing of 137 patients in Shaanxi has indicated that the Delta strain of the virus is similar to imported cases from an international flight from Pakistan that arrived in Xi'an on Dec 4.

Ma said more confirmed cases were expected as the province expands nucleic acid testing and screening in the days to come.

He urged residents to actively cooperate in the fight against the pandemic and not to panic.

He also urged Xi'an residents not to leave the city if unnecessary.

Wei Xiaoli, deputy director of the Xi'an center for disease control and prevention, said the sharp increase in the number of confirmed cases detected in the previous two days was due to the increase of nucleic acid tests.

Zhang Boli, an academician at the Chinese Academy of Engineering, forecast the Xi'an outbreak might reach a turning point in the second half of January and be put under control before the Chinese New Year in early February.

Moreover, he urged relevant departments in Xi'an to go all out to prevent the epidemic from spreading to rural areas and beyond the city.

Shi Bingyin, a professor with the First Affiliated Hospital of Xi'an Jiaotong University, said most of the patients in this round of outbreak have only mild symptoms or are asymptomatic carriers.

That indicated the domestic vaccines have played an effective role, he added.

Kang Ruoming contributed to this story.