outh Korea reported on Saturday its biggest surge in new coronavirus cases and concerns grew of a possible epidemic in the United States as the World Health Organization raised its risk alert to its highest level.

The virus has rapidly spread across the world over the past week, causing stock markets to sink to their lowest levels since the 2008 global financial crisis over fears that the disease could wreak havoc on the world economy.

More than 2,900 people have died and over 85,000 have been infected worldwide since it emerged apparently from an animal market in the central Chinese city of Wuhan in December.

The vast majority of infections have been in China but more daily cases are now logged outside the country, with South Korea, Italy and Iran emerging as major focal points.

South Korea has the most cases outside China, with nearly 3,000 infections as 594 more patients were reported on Saturday -- the country's biggest increase to date.

In neighbouring North Korea, leader Kim Jong Un warned top party officials of the "serious consequences" of failing to prevent an outbreak of the new coronavirus on their side of the border.

The impoverished nation, with a weak and ill-equipped healthcare system, has closed its borders to prevent the spread of the disease into its territory.

The virus has also spread to new zones -- in the past 24 hours, it has affected nine new countries, from Azerbaijan to Mexico and New Zealand after reaching sub-Saharan Africa with Nigeria reporting its first case.

"We have now increased our assessment of the risk of spread and the risk of impact of COVID-19 to very high at global level," WHO chief Tedros Adhanom Ghebreyesus told reporters on Friday.

"We do not see evidence as yet that the virus is spreading freely in communities. As long as that's the case, we still have a chance of containing this virus."

But US health officials on Friday reported two more cases of the new coronavirus transmitted to people who did not travel overseas or come in contact with anyone known to be ill, indicating the disease was spreading in the country.

There are now three such cases in the United States, with one in Oregon and two in California, in addition to some 60 other infections in the country.

"This new case indicates that there is evidence of community transmission but the extent is still not clear," said Sara Cody, director of public health for California's Santa Clara County, the heart of Silicon Valley where tech giants like Apple and Google are based. "What we know now is that the virus is here, present at some level, but we still don't know to what degree."

- Economy, events hit -

US Federal Reserve chair Jerome Powell said the central bank stood at the ready to intervene if needed, given the "evolving" risks to the world's largest economy posed by the outbreak.

Official data released in China on Saturday showed the extent of the damage caused to the world's second-largest economy, with manufacturing activity falling to its lowest level on record in February as key industries ground to a standstill under drastic containment measures.

More global events were disrupted due to the epidemic, with the United States delaying a regional summit of the Association of Southeast Asian Nations (ASEAN) due in Las Vegas next month.

"This is not a time for panic. It is time to be prepared -- fully prepared," UN Secretary-General Antonio Guterres said.

Those efforts come as the number of deaths and new infections has been tapering off in China, following unprecedented quarantine efforts locking down tens of millions of people in the worst-hit cities. But they have kept rising abroad.

- South Korean cases -

South Korea's epidemic is centred in its fourth-largest city, Daegu, whose streets have been largely deserted for days, apart from long queues at the few shops with masks for sale.

Three women in the Daegu area died of the illness, taking the national toll to 16, authorities said Saturday.

South Korea's total is expected to rise further with screening of more than 210,000 members of the Shincheonji Church of Jesus, a secretive entity often accused of being a cult that is linked to around half of the country's cases.

While China reported 47 more deaths on Saturday, it recorded fewer new cases than South Korea, with 427 infections.

South Korean officials say they are not considering a citywide quarantine for Daegu similar to the lockdown imposed on the central Chinese city of Wuhan.

In Iran, unnamed health system sources told the BBC that at least 210 people had died of the coronavirus -- far beyond the official death toll of 34, but a health ministry spokesman angrily denied that figure.

Meanwhile, China on Saturday reported 47 more deaths from the new coronavirus, raising the death toll in the country to 2,835. The National Health Commission also tallied 427 new cases, an increase from the previous day, bringing the total number of infections to 79,251.

One person died in Beijing and another in Henan province, while 45 died in Hubei.

All but four of the new cases were in Hubei, and 420 were in its capital, Wuhan, where the virus is believed to have originated in a market that sold wild animals.

While the number of new cases rose from 327 on Friday, they remain in the hundreds, far below the huge, daily increases that China was recording more than 10 days ago.

The World Health Organization said earlier this week that there were now more daily cases abroad than in China.

China has placed 56 million people in Hubei under strict quarantine for more than a month and limited crowds across the country in an unprecedented effort to contain the virus.

Officials say the country has made progress against the virus but measures remain in place, including the closure of schools. Businesses have slowly started to reopen.

US health officials reported a third case of the new coronavirus transmitted to a person who did not travel overseas or come in contact with anyone known to be ill, indicating the disease was spreading in the country.

Authorities said the new case concerned a person living in the western state of Oregon. The adult patient, who has been hospitalized, was known to have had contact with people at an elementary school.

Oregon Health Authority director Patrick Allen told reporters that the case is considered "presumptive" pending confirmation of the test results from the Centers for Disease Control and Prevention. "We've been expecting this and we're prepared for it," he said.

Officials said the elementary school will be shut down until March 4 as it undergoes a deep clean and as health officials talk to employees and parents.

Two other "community spread" cases have been reported in neighboring California this week.

Both cases involve older people in the northern part of the state who mysteriously contracted the virus.

The increasing number of people stricken with the illness in the United States is sure to heighten fears of an outbreak across the country.

According to the Centers for Disease Control, as of Thursday there were more than 60 people infected with the disease in the United States.

California's Governor Gavin Newsom on Thursday said 33 people there had tested positive for the virus, and five of them had left the state.