TOKYO (REUTERS) - Yokohama voters were casting ballots for mayor on Sunday (Aug 22) in an election expected to affect Prime Minister Yoshihide Suga's prospects for keeping his post amid a Covid-19 surge across Japan.

A loss by the Suga-backed Liberal Democratic Party (LDP) candidate in Mr Suga's home turf just south of Tokyo would pile pressure on the premier, whose approval ratings slid below 30 per cent in August, fanning concerns in the LDP about his ability to lead into a general election this year.

Mr Suga, who took office last September after predecessor Shinzo Abe quit citing ill health, is struggling to contain the pandemic, with national daily Covid-19 infections hitting a record 25,000 last week.

Mr Hachiro Okonogi, backed by Mr Suga, is running in a crowded field of eight, including the incumbent mayor, two former governors and opposition Constitutional Democratic Party of Japan candidate Takeharu Yamanaka, a professor of public health.

Although Yokohama is in Mr Suga's constituency in Parliament, a survey conducted by a local newspaper in mid-August found Mr Okonogi trailing Prof Yamanaka, reflecting concerns over the government's handling of the pandemic.

Kanagawa, the prefecture where Yokohama is located, has been under a state of emergency since Aug 2, but infections have continued to rise. The prefecture recorded a record 2,878 cases last Friday, according to public broadcaster NHK.

Mr Suga's term as LDP president ends in September. A general election must be held by Nov 28.