Four more people have died in the US state of Washington after being infected with the coronavirus, bringing the death toll in the state and country to six, authorities said on Monday.

Dr Jeff Duchin, chief health officer for the Seattle and King County Public Health agency, announced the increase in fatalities from COVID-19 at a news conference on Monday, as authorities across the country scrambled to prepare for more infections with an emphasis on ratcheting up the number of available test kits.

Researchers had previously said that the virus may have been circulating undetected for weeks in Washington state. After the first death, Governor Jay Inslee declared a state of emergency.

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Eight of the 18 total cases in the state and four of the deaths are linked to a nursing facility in the Seattle area, Duchin said. At least four of the six people who have died were elderly or had underlying health conditions, he added.

"We expect the number of cases will continue to increase in the coming days and weeks, and we are taking this situation extremely seriously," he said, adding that, like in most of the world, the vast majority of diagnosed patients have mild to moderate disease and do not require hospitalisation.

As of Sunday, the number of cases in the US had risen to 91, the US Centers for Disease Control and Prevention (CDC) said earlier on Monday. So far, 10 states, including California, New York, Oregon, Florida, and Rhode Island have confirmed or presumed coronavirus cases.

In New York, Governor Andrew Cuomo, whose state has one confirmed case, welcomed a CDC decision that allows state authorities to test for the virus, vastly reducing testing time.

"I would like to have a goal of 1,000-tests-per-day capacity within one week because, again, the more testing, the better," Cuomo said at a briefing on Monday.

Since the virus first appeared in the city of Wuhan, China, in December it has infected over 89,000 people worldwide, killing over 3,000.

US government prepares

Government military laboratories have been working to develop a vaccine for virus, the chairman of the Joint Chiefs of Staff, Army General Mark Milley, told a news conference on Monday.

Meanwhile, President Donald Trump has said that his administration has asked pharmaceutical companies to accelerate work on the development of a coronavirus vaccine, but provided no details.

Top US health officials have said any vaccine is up to 18 months away.

There is currently no treatment for the respiratory disease, although patients can receive supportive care.

The US Congress is expected to take up a spending measure in coming days that could allocate billions more dollars for the virus response.