Measures are being stepped up amid a rising caseload of COVID-19 in the United States, with a "stay at home order" issued in California, a southwestern state with nearly 40 million people, to curb the further spread of the virus.

California Governor Gavin Newsom on Thursday issued the statewide order that restricts residents' nonessential activities, two days after a "shelter in place" order was imposed in counties in the Bay Area of Northern California, affecting 7 million people.

"We are confident the people of California will abide by it; they will meet this moment," he said at a news conference from the state capital Sacramento, when announcing the unprecedented, sweeping public health order that went into effect immediately.

Newsom said the decision is based on modeling projections that 56 percent of the state population will get infected over the next eight weeks, with a gap of about 20,000 hospital beds for coping with the situation, and with Los Angeles, the second-largest US city, to be severely hit.

He assured residents that they "can still take your kids outside, practicing common sense and social distancing. You can still walk your dog". Restaurant meals can still be delivered to homes.

Data released by the Center for Systems Science and Engineering at the Johns Hopkins University showed that more than 14,200 cases had been reported by 10:13 pm local time in the United States, with 205 deaths. The tally indicated more than 500 cases were confirmed in less than an hour.

Also on Thursday, US President Donald Trump called off a June meeting of G-7 nations at Camp David due to the pandemic. Instead, those leaders will meet by videoconference.

The pandemic has so far claimed 18 lives in California, while nationwide, Washington state recorded the biggest death toll of 74.

Most sweeping measure

The stay-at-home order, deemed the most sweeping government public health measure undertaken in the US, came after San Francisco required residents to shelter in place beginning on Tuesday through to April 7. That order was enacted in San Francisco and the counties of Santa Clara, San Mateo, Marin, Contra Costa, Alameda, requiring residents to stay at home except for essential activities.

San Francisco Mayor London Breed said the measures "are absolutely necessary" to slow the spread of the virus in the community.

"We all have a responsibility to do our part to protect our neighbors, and slow the spread of this virus by staying at home," he said.

With bans on public gatherings, and the closure of cinemas, theaters, bars, restaurants, and schools amid a growing public health crisis, it is likely that other US states and cities will follow the example of California to take more aggressive measures to prevent the further spread of the virus.

On Tuesday, New York Mayor Bill de Blasio said the city was considering introducing measures like those enacted in San Francisco, with a decision to be made within days.

New York Governor Andrew Cuomo on Tuesday called for national unity in the face of the pandemic as the state became the first in the US to report more than 1,000 cases. So far in New York City, 1,871 people had tested positive on Thursday, compared with 814 on Tuesday.

"Everybody is afraid. Everybody is nervous. How you respond, how you act, this is a character test for all of us individually. It is a character test for us collectively as a society," Cuomo said.

As anti-virus measures are being toughened in a number of states, the US State Department on Thursday raised its travel advisory to Level 4, which instructs US citizens to avoid all international travel.

Xinhua, agencies and Ai Heping in New York contributed to this story.