(Changes "up" to "down" in paragraph 2)

BEIJING, Sept 8 (Reuters) - Chengdu, capital of the southwestern Chinese province of Sichuan, extended the lockdown of most of its districts on Thursday, hoping to stem further transmission of COVID-19 cases in the city of 21.2 million.

The local government reported 116 new local cases, down from 121 a day ago. Of the cases reported, 57 were symptomatic and 59 were asymptomatic.

The lockdown was expected to be lifted on Wednesday, but local government officials took extreme caution, saying "there are still risks of social spread in some areas," according to Chengdu authorities.

Residents under lockdown will be tested for the virus every day, Chengdu authorities said late on Wednesday, without giving a date on when the lockdown would be lifted.

Residents of a handful of districts that were released from a full lockdown will undergo mass testing on Sept. 9 and Sept. 11.

The mega city locked down last week as cases rose across several districts, allowing authorities to complete another round of mass testing. About 90% of flights at Chengdu's Shuangliu Airport were cancelled last Monday.

China this year has been battling to contain the highly transmissible Omicron variant, imposing various degrees of lockdown on cities to stop its spread.

Shanghai, locked down for two months in April and May, was one of the more prominent cities that were affected by China's so-called "dynamic zero-COVID policy", where infections are to be stamped out when they emerge. (Reporting by Beijing newsroom; Writing by Bernard Orr; Editing by Gerry Doyle)