San Francisco officials are putting the city into an effective lock-down for three weeks as the state grapples with the coronavirus pandemic.

Similar orders have been put in place in Alameda, Contra Costa, Marin, Santa Clara and San Mateo counties in California's Bay Area, impacting more than 6.7m people.

San Francisco's citywide public health order will prohibit residents from leaving their homes except to visit doctors and buy groceries and medicine through at least 7 April, Mayor London Breed announced on Monday. The orders are effective at midnight.

She said: "This is going to be a defining moment for our city ... We all have a responsibility to do our part to protect our neighbours and slow the spread of this virus by staying at home unless it is absolutely essential to go outside."

The orders mark the most dramatic local responses yet to the Covid-19 outbreak with the US, mirroring plans across Europe and Asia on the frontline of the pandemic.

San Francisco police will enforce the "shelter-in-place" orders. Police chief Bill Scott said that residents' failure to comply will be a misdemeanour offence.

People experiencing homelessness are encouraged to check into local shelters.

San Francisco's public health director Grant Colfax said: "The virus is here in San Francisco. We must practice social distancing to slow it down ... Every hour counts."

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Health officials have reported at least 251 cases in the Bay Area, with more than half of those cases confirmed within the last several days. Forty cases have ben reported in San Francisco, and four people in the region have died.

But health officials across the US have warned that testing shortfalls mean that the confirmed diagnoses mask a greater undiagnosed population.

More than 4,000 people in the US are confirmed to have the virus, and 73 people have died.

California Governor Gavin Newsom also has ordered the closure of all bars and restaurants throughout the state, mirroring similar actions in cities and states across the US.

The Centers for Disease Control and Prevention has recommended that people avoid gatherings with more than 50 people, though the White House's latest guidance tells Americans to avoid groups of 10 or more.

Mayor Breed has also announced a $10m plan to provide partial paid sick leave for workers impacted by the shut-down.

That sum provides businesses with five days of sick leave pay beyond the company's existing policies. Twenty per cent of the funds are reserved for small businesses with 50 or fewer employees.

The city will pay out a minimum wage of $15.59 per hour, or $623, per employee. The employer will pay the difference between the minimum wage and an employee's full hourly wage.