SACRAMENTO (REUTERS) - Nearly half of all Californians live in areas where coronavirus infections and hospitalisations are rising quickly enough to put their counties on a watch list for potential reinstatement of shutdowns, a Reuters analysis of state data shows.

More than 18 million of the most populous US state's 39 million residents live in counties where rates of increase have put them on a watch list that may eventually require them to roll back reopening efforts, the Reuters analysis shows.

Overall, there were 133,489 cases in California by Tuesday (June 9), and nearly 4,700 deaths.

Driven in part by increased social gatherings and workplace transmissions, state data updated on Tuesday showed rising transmission and hospitalisation in nine counties, including populations centres like Los Angeles, Sacramento and Fresno counties, as well as the more rural Tulare and Imperial counties.

"Many of the cases that are showing up in hospitals are linked to gatherings that are taking place in homes - birthday parties and funerals," said Olivia Kasirye, public health director of Sacramento County.

One cluster in the state capital region was caused by a single traveller: "Someone travelled from another state and came to visit relatives, and then they all started falling sick," Kasirye said.

New diagnoses in the heavily populated Los Angeles area are going up in part because testing is more widely available. But officials say infections and hospitalisations in most other parts of the state are being driven by factors tied directly to the loosened restrictions or overt flouting of public health rules.

The rising concern in California comes as 21 US states reported weekly increases in new cases of Covid-19, with Arizona, Utah and New Mexico all posting rises of 40 per cent or higher for the week ended June 7 compared with the prior seven days, according to a Reuters analysis.