The Chinese city of Xi’an recorded 175 local coronavirus infections on Monday, as it grapples with a coronavirus outbreak that has pushed China’s daily cases to record highs since early last year.

The 175 infections reported in Xi’a, in the northwestern Shaanxi province, were up from 162 on Sunday, and 158 the day before, according to a government notice issued Tuesday. Authorities blame the outbreak on the Delta variant.

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Authorities have locked down 13 million residents in Xi’an amid the rise in infections as they attempt to curb the spread. China is one of the few remaining countries with a “zero COVID” policy, with authorities implementing mass-testing when infections are found in the community.

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Xi’an has so far conducted four mandatory rounds of testing as authorities sought to detect new infections. The city has also launched a disinfection campaign, spraying down roads and buildings.