Seven counties in California's Bay Area have extended stay-at-home orders through the end of May, urging that the area's 7 million residents continue to shelter in place through the coronavirus pandemic.

The counties were the first among local governments in the US to put in place quarantine measures before California Governor Gavin Newsom had issued a statewide order.

Alameda, Contra Costa, Marin, San Francisco, San Mateo and Santa Clara counties and the city of Berkeley will ease some restrictions on "lower-risk activities" under the new orders. Further details will be announced later this week.

The initial stay-at-home orders were set to expire on 3 May.

In a joint statement issued by the counties' public health directors, the area has made 'substantial progress in slowing the spread of the novel coronavirus, ensuring our local hospitals are not overwhelmed with Covid-19 cases, and saving lives" but stressed that at this stage of the public health crisis "it is critical that our collective efforts continue so that we do not lose the progress we have achieved together."

Though hospitalisations have levelled, "more work is needed to safely re-open our communities" and "prematurely lifting restrictions could easily lead to a large surge" in new cases, the statement says.

Health officers will release a "set of broad indicators that will be used to track progress in preparedness and response" to the outbreak.

The next phases of easing restrictions will require that "each jurisdiction and various sectors continue to rapidly build critical infrastructure and systems to respond to and control the spread of coronavirus infections and to ensure the health care system's ability to meet demand," the statement says.

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"The virus spreads easily, testing capacity is limited and expanding slowly, and vaccine development is just beginning," it adds. "We expect to be responding to Covid-19 in our communities for a long time. As effective as our efforts have been, if we move too fast to ease restrictions, the potential of exponential spread could have grave impacts to health and wellness of our residents as well as the economy."

State health officials have identified more than 43,000 cases of the virus, including at lease 1,755 deaths.

More than 3,300 people have been hospitalised, including 1,185 people in intensive care units.