The World Health Organisation raised its global risk assessment for the new coronavirus to "very high" from "high" as some countries struggle to contain the pathogen.

Infections that are spreading from Italy and Iran to other countries are raising concerns about the virus that emerged in China, WHO Director-General Tedros Adhanom Ghebreyesus said in a briefing on Friday. Some health systems aren't ready to deal with the widening epidemic, he said.

Elevating the risk level is meant to spur countries to step up their responses, said Mike Ryan, who runs the agency's emergencies program. Health officials need to be ready to isolate patients and carefully trace their contacts to keep the disease from spiralling out of control, he said.

"This is a reality check for every government," he said. "The window of opportunity is closing. Wake up."

Health officials are trying to contain the virus, which can cause Covid-19, a pneumonia-like illness that can be severe in a minority of patients and transmitted from others who look healthy. After it's reached more than 50 countries in about two months, some researchers and scientists are at the edge of calling the disease a pandemic, a global, potentially lethal outbreak.

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Ryan said it would be unhelpful to describe it as a pandemic because that would imply that everyone in the world will get it. Countries including China, Singapore, Nepal and Vietnam have shown that containment measures can work, the WHO said.

"We can avoid the worst of this, but our level of concern is at its highest," Ryan said. "A lot can be done to fight the virus."

A research team convened by the agency released a report on Friday showing that transmission of the coronavirus in China has largely occurred within families. That suggests that measures like China's massive quarantine of some 50 million people can work to prevent the illness from spreading in the community, beyond the view of health workers who can take steps to isolate infected people and quarantine their contacts.

Italy has instituted a lockdown in the northern regions of Lombardy and Veneto, where it has detected more than 600 cases. Still, Nigeria reported its first confirmed case of the disease after an Italian citizen took a commercial flight from Milan to Lagos.

Health officials have cautioned that the export of the disease to poor countries could challenge their health systems and breed a much wider epidemic. Philanthropist and Microsoft co-founder Bill Gates has warned the impact in sub-Saharan Africa could be "very, very dramatic", the Telegraph reported.

Wealthy countries can help slow the virus by helping poorer nations prepare for a pandemic, Gates said in an article in the New England Journal of Medicine on Friday. By helping African and South Asian countries get ready, nations can save lives and slow the circulation of the virus, Gates said.

The coronavirus "has started behaving a lot like the once-in-a-century pathogen we've been worried about", he said. "I hope it's not that bad, but we should assume it will be until we know otherwise."

The WHO in late January declared the outbreak a public health emergency of international concern, a designation that sent an important message about its severity. At the time, the WHO didn't recommend restrictions on travel or trade, but many countries and companies have taken such steps.