Residents of Beijing's medium and high-risk areas, as well as those in the lockdown zones and controlled zones, should not leave the city in order to prevent the virus transmission, senior official said on Tuesday.

The announcement came as the number of COVID-19 cases has risen since April 22 to 453 by Tuesday.

The same policy applies to those who live in the communities, towns and villages where one or more confirmed COVID-19 cases have been reported, said Tian Wei, an official from Beijing's information office, at a news conference on Tuesday afternoon.

"People who have received risk alert from Beijing Jiankangbao - a mobile-based app used to check health codes and provide nucleic acid testing services, should not leave Beijing," he added.

Other residents should not leave the city if the trip is not absolutely necessary. Those who have to leave the city should first have negative nucleic acid test results within 48 hours before boarding.

After the five-day May Day holiday, workers should have negative nucleic acid test results taken within 48 hours before returning to work.

Li Yi, spokesperson of Beijing's education commission, said on Tuesday that students of kindergartens, primary and middle schools would delay return date for a week to May 11 and hold online courses during the period of time. The exact return date will be decided based on the epidemic situation.

"The students will start online courses after the holiday," he said. "The schools have gained lots of experience during the past two years' fight against the epidemic that they will provide enough guidance to students in both studying and mental health."