China is starting to reimpose COVID-19 restrictions just weeks after major easing in key cities, raising concern the country may once again employ strict lockdowns to control its outbreak.

Beijing reported 51 new local cases for Sunday, after having single digit cases on most days last week. The city’s local government said an outbreak linked to a popular bar is proving more difficult to control than previous clusters, in a weekend that saw mass testing and rising infections both in the capital and Shanghai. A total of 37 cases were reported for Shanghai, with five new cases detected in the community.

Authorities delayed the reopening of most schools in Beijing that was planned for Monday, while most districts in Shanghai suspended dine-in services at restaurants. China reported 143 cases nationwide for Sunday as the number of new infections crept up. Daily cases fell below 100 last month for the first time since early March after strict curbs.