Washington state - epicentre of the US's coronavirus crisis - has become the first in the nation to prohibit gatherings of more than 250 people as part of an effort to control the disease's spread.

As a top regional health official warned they were expecting a “major outbreak” of the the disease “within weeks”, governor Jay Inslee issued an executive order that will impact sporting, social, musical and religious events.

He also warned schools in the state to prepare contingency plans for possible shutdowns that could last weeks or months, and said he suspected other states would follow the measure ordered in the Pacific Northwest, where most of the 30-odd coronavirus deaths have taken place.

He said if these measures of “social distancing” were not enacted, estimates suggested as many as 60,000 people could be infected with COVID-19 by the middle of May “with the resultant fatalities”.

Asked if he considered the measures being introduced we “draconian”, he replied: “I don’t think it’s draconian to try and stop another 22 people losing their grandfather.”

The measures announced by Mr Inslee for King, Pierce and Snohomish counties located in and around Seattle, are the most stringent yet declared by non-federal authorities.

He said they had been introduced after he spoke to vice president Mike Pence, who is heading the Trump administration’s response to the virus, which has infected more than 1,000 people in the US and killed around 30. He said he suspected similar measures might be recommend by the federal government.

Jeff Duchin, the health officer for Seattle and King County, said officials were doing what they could to try and limit the spread of the virus, though he admitted there were limitations to what could be done.

“We don't take the decisions lightly. We're facing an unprecedented health emergency,” he said.

“We expect a large scale outbreak in weeks and this will be a very difficult time. It's similar to what you might think of as an infectious disease equivalent of a major earthquake that's going to shake us for weeks and weeks.”

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Among those already responding to the order is the Mariners baseball club, one of the region’s many sporting teams.

The club announced it would be moving home games out of Seattle until the end of March.

Seattle had been scheduled to open the season at T-Mobile Park with a four-game series against Texas from March 26-29, then host Minnesota in a three-game series from March 30 through April 1.

The Mariners said Wednesday they are working with the commissioner’s office on alternative plans.

“While we hope to be back to playing baseball in Seattle as soon as possible, the health and safety of our community is the most important consideration,” the team said in a statement.