SHENZHEN, China, Aug 13 (Reuters) - China's most populous city Shanghai has extended its weekly COVID-19 test requirement and extended free testing until the end of September in a bid to keep the virus in check, authorities announced on Saturday.

Citizens without a record of a nucleic acid test from within seven days will be assigned a yellow code on Shanghai's health code system, the official notice said. A yellow code restricts access to some public venues.

On Saturday health authorities in the financial hub of 25 million said they detected one symptomatic and three asymptomatic cases a day earlier.

The southern province of Hainan is currently China's worst hit region, with 594 symptomatic cases and 832 asymptomatic cases reported on Saturday for 24 hours earlier.

Tight restrictions and lockdowns in the popular tourist destination are expected to last over the weekend.

On Saturday China's National Health Commission reported a total of 2,144 new COVID-19 infections on Aug. 12, of which 704 were symptomatic and 1,440 were asymptomatic.