Fake flyers telling diners to avoid Asian-American restaurants because of the coronavirus are among a spate of recent racist incidents linked to the outbreak, say California authorities.
Coronavirus fears have spread even though the US has seen just 15 cases, over half in California.
This week in Los Angeles bullies accused an Asian-American student of having the virus and badly beat him.
The coronavirus has now reached 24 countries outside China.
Anxiety and misinformation related to the virus have fuelled anti-Asian prejudice, Los Angeles authorities said at a press conference.
"Many may be quick to assume that just because someone is Asian or from China that somehow they are more likely to be carriers of the virus," said Robin Toma, executive director of the LA County Human Relations Commission.
A Los Angeles Asian-American schoolboy accused by bullies of having the virus was taken to the hospital after being beaten.
"He went to the hospital originally and went to the emergency room," Mr Toma said. "They were taking MRIs to ensure he didn't have a concussion or other harm."
There has been just one case of the virus reported in Los Angeles County, which has a population of 10.1 million.
But in the Los Angeles area, flyers with counterfeit seals for the World Health Organization (WHO) have been posted.
They advised residents to avoid Asian-American businesses like Panda Express because of the coronavirus.
In the nearby Alhambra area, 14,000 people have signed a petition urging school closures over the virus.
And in a now-deleted Instagram post on "managing fears and anxiety", the University of California, Berkeley health services department listed xenophobia as a "normal" reaction amid a virus outbreak.
The prejudicial attacks could worsen given the possibility that the virus will spread in US communities in the coming weeks, said Mr Toma.
Similar incidents have been reported worldwide, including Canada, the UK and France.
But coronavirus cases are not rising dramatically outside China, the WHO has said.
There was also no major shift in the coronavirus' pattern of mortality or severity, according to the WHO.