US President Donald Trump has offered China "any help that is necessary" to control a coronavirus outbreak that has killed 81 people and left tens of millions stranded on the biggest holiday of the year.

Chinese Premier Li Keqiang will "inspect and direct" efforts to control the outbreak and promised reinforcements, as provincial authorities faced accusations they failed to respond in time.

World shares slipped to their lowest in two weeks as worries grew about the economic impact of the virus after China, the world's second-biggest economy, ramped up travel bans and extended the Lunar New Year holidays.

"We are in very close communication with China concerning the virus," said a Twitter post from Trump, who waged a bruising 18-month trade war with Beijing.

"Very few cases reported in USA, but strongly on watch. We have offered China and President Xi [Jinping] any help that is necessary. Our experts are extraordinary!"

Li, clad in a blue protective suit and mask, thanked medical workers in Wuhan, the capital of the central Hubei province and the epicentre of the outbreak.

"Li ... thanked frontline medical workers for their all-out efforts in treating patients and urged them to pay attention to their own protection," Xinhua news agency said.

"He required efforts to guarantee medical resources supply, race against time to treat patients and ensure adequate market supply and stable prices."

Li said 2500 more medical workers would arrive in the next two days.

Li is the most senior leader to visit Wuhan since the outbreak began. He inspected efforts to contain the epidemic and was shown on state television leading medical workers in chants of "Wuhan jiayou!" - an exhortation to keep their strength up.

He also visited the construction site of a new hospital due to be built in days.

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On China's heavily censored social media, where dissent is typically suppressed, local officials have borne the brunt of mounting public anger about the handling of the virus.

Some lashed out at the Hubei governor, who had to correct himself twice during a news conference over the number of face masks being produced in the province.

"If he can mess up the data multiple times, no wonder the disease has spread so severely," one Weibo user said.

Wuhan Mayor Zhou Xianwang told state broadcaster CCTV the city's management of the crisis was "not good enough" - rare public self-criticism for a Chinese official - and said he was willing to resign.

The city of 11 million people is in virtual lockdown and much of Hubei, home to nearly 60 million people, is under some kind of travel curb.

A small number of cases linked to people who travelled from Wuhan have been confirmed in more than 10 countries, including Australia, Thailand, France, Japan and the United States, but no deaths have been reported outside China.

Asian and European shares tumbled, with Japan's Nikkei average sliding 2 per cent, its biggest one-day fall in five months.