China's northern coastal city of Tianjin tightened restrictions on Sunday after over 40 COVID-19 cases, including two local cases of the Omicron variant, were detected.

Citizens have been advised against leaving the municipality, which is about 120 kilometers away from the capital Beijing. A negative PCR test within 48 hours and a green health code are required for those who wish to leave.

The Tianjin municipality, which has a population of 13.86 million people, decided to carry out citywide nucleic acid testing after 20 people tested positive for COVID-19 on Saturday.

An additional 20 positive COVID-19 cases were detected in the latest screening between 9 p.m. Friday to 9 p.m. Sunday, local health authorities said on Sunday night.