Washington State Governor Jay Inslee (C) addresses the press during a visit by Vice President Mike Pence to discuss concerns over the coronavirus.

Public officials on Wednesday banned large gatherings in San Francisco and the Seattle area as cities seek to curb the spread of the new flu-like coronavirus that's infected more than 121,000 people across the world since December.

Washington Gov. Jay Inslee said he's banning gatherings of more than 250 people in several counties across the state to try to contain the COVID-19 outbreak that that has killed at least 23 residents. Inslee's order applies to King County, which encompasses Seattle, as well as nearby Snohomish and Pierce counties. About 4 million people live in the impacted area.

"The number of cases doubles every several days and we've had multiple instances of COVID-19 in long-term care facilities where vulnerable persons reside. We expect a large-scale outbreak in weeks and this will be a very difficult time," King County health officer Dr. Jeff Duchin said at a news conference with Inslee.

COVID-19 cases surpassed 1,000 in the U.S. late Tuesday, up nearly tenfold from a week ago. Earlier Wednesday, the World Health Organization declared the COVID-19 outbreak a global pandemic.

Moments after Inslee's announcement, San Francisco health officials announced they were prohibiting public and private events of 1,000 or more to slow the spread of the new coronavirus there.

"We know that this order is disruptive, but it is an important step to support public health," San Francisco Mayor London Breed said in a statement. "We know cancelling these events is a challenge for everyone and we've been talking with venues and event organizers about the need to protect public health." She said she spoke with the Warriors NBA team and "they are in support of our efforts."

The San Francisco ban is for two weeks, the mayor's office said in a statement, adding that the city's public health officer can extend it.

Inslee said the canceled events in Washington include community, civic, faith-based, sports and other events. In response, the Seattle Mariners of MLB said they were considering "alternative plans" for their games scheduled for the end of March.

Washington state is the hardest hit in the U.S. with more than 267 confirmed cases across the state — 258 of which are concentrated in the three counties, according to the state health commission. Inslee declared a state of emergency last month to free up funding for communities combating the outbreak.

"This is an extremely dangerous event that we are facing, but we are not helpless," Inslee said. "All it requires is the will of active people to follow science and confidence that we're all in this together."

The ban in Washington, which lasts through the rest of March, could be expanded to other counties and will likely be extended beyond March, he said.

"We hope that the things we're doing aggressively here can set a template for the rest of the country," he said.

King County, which is the hardest-hit part of Washington, took the order a step further. Duchin issued an order prohibiting events with fewer than 250 people unless the organizers "can take steps to minimize the risk to participants." He added that the order is not intended for retail establishments or family events.

Seattle-area officials announced Tuesday that COVID-19 has infected residents or employees of 10 long-term care facilities. So far, at least 19 of the deaths in the U.S. are tied to one such facility: The Life Care residential home in Kirkland, Washington.

The actions, officials said, will hopefully slow the spread of the virus to ensure that the local health-care system is not overwhelmed.

"Regardless of what we do the health-care system will be very stressed," Duchin said. "This outbreak will go on for weeks and likely for months."

Clarification: The story has been updated to clarify that Washington state's order currently applies to three counties, not the entire state.