Shanghai, China's most populous city, on Tuesday again tightened the first phase of a two-stage COVID-19 lockdown, asking some residents to stay indoors unless they are getting tested as the number of daily cases rose beyond 4,400.

China's financial hub, home to 26 million people, is in its second day of a lockdown local authorities are carrying out by splitting the city roughly along the Huangpu River, dividing the historic centre from the eastern business and industrial district of Pudong to allow for staggered testing.

While the Shanghai caseload remains modest by global standards - a record 4,381 asymptomatic cases and 96 symptomatic cases for March 28 - the city has become a testing ground for the country's "zero-COVID" strategy as it tries to bring the highly infectious Omicron variant under control.