LOS ANGELES (BLOOMBERG) - Los Angeles has a new tool to dissuade large parties that double as virus super-spreader events: shutting off offending properties' electricity and water.

Mayor Eric Garcetti said that he has authorised the city to cut water and power in "egregious cases" of houses, businesses and other venues hosting large gatherings, effective from Friday (Aug 7) night.

"We've seen the reports of some large parties and gatherings in flagrant violations of health orders," he said in a press conference late Wednesday. "These house parties have essentially become nightclubs in the Hills."

The order follows a gathering this week that reportedly attracted hundreds of people at a mansion on Mulholland Drive and ended in gunfire. The event and others have raised concern that infections will accelerate in the epicentre of California's Covid-19 outbreak.

Los Angeles County has been making some progress in containing the virus, which has infected more than 197,000 residents.

Public Health Director Barbara Ferrer said on Tuesday that there is cautious optimism "that our efforts over the past few weeks may be starting to slow the spread".

The county on Wednesday reported 68 additional deaths and 2,347 new cases. It said nearly 60 per cent of new Covid-19 cases occur in residents aged between 18 and 49.

"All the sacrifices can be undone by those who refuse to follow the science and follow the rules - and put our economy and our community at risk," Mayor Garcetti said.

Elsewhere, New York and New Jersey - early hot spots of the pandemic - are getting tough on parties, anti-maskers and out-of-town visitors that are raising the risk of a resurgence.

New Jersey's indoor-gathering limit was cut by 75 per cent on Monday, to 25 people, after a spate of house parties led to almost 100 Covid-19 cases.