The US is in close contact with China and offers whatever help is needed to fight the coronavirus outbreak, President Donald Trump said on Monday, as the US Centers for Disease Control and Prevention (CDC) raised its travel precautions for the country to its highest level.

"We are in very close communication with China concerning the virus," Trump tweeted early Monday. "Very few cases reported in the USA, but strongly on watch. We have offered China and President Xi any help that is necessary. Our experts are extraordinary!"

Vice-President Mike Pence echoed the president in the afternoon, saying at the White House Initiative on Asian Americans and Pacific Islanders Lunar New Year Celebration that he knew the hearts of many Americans "are with the Chinese families that are being impacted" by the viral outbreak.

"We stand ready to help the people of China confront the coronavirus," Pence said.

So far, 4515 cases of pneumonia caused by the novel coronavirus (2019-nCoV) were confirmed in China, including 976 people in critical condition reported in the country by the end of Monday. The death toll exceeded 100, and China extended the traditional Lunar New Year holiday to keep the public at home and avoid spreading infection.

In the US, five travelers from Wuhan have been confirmed to have been infected with the virus in Arizona, California, Illinois and Washington state.

The situation has prompted the US State Department to encourage Americans to "reconsider" any plans to travel to China on Monday.

On Sunday, the US State Department said it will organize a single flight to evacuate diplomats and a few private citizens from Wuhan where the virus broke out last month. The plane is expected to leave Tuesday and fly to San Francisco.

Also on Monday, the CDC raised its travel precautions for China to its highest of three levels: "Warning". "CDC recommends that travelers avoid all nonessential travel to China," it said in an update.

The CDC tweeted earlier in the day, "The immediate health risk from #2019nCoV in the U.S. is currently considered low."

"Person-to-person spread in the United States has not yet been detected, but it's likely to occur to some extent," the CDC said in an advisory posted to its website.

Nevertheless, the CDC said more cases in the US are "likely" to be detected, including in the US. The virus could be spread person-to-person and it "would not be surprising" if the virus were spread in some hospitals.

The CDC is screening passengers on direct and connecting flights from Wuhan at five major airports: Atlanta, Chicago, Los Angeles, San Francisco and New York.

CDC teams have been sent to Illinois and Washington state to assist in ongoing investigations of the virus.

The CDC recommends getting a flu shot and taking routine preventative actions such as frequent and vigorous hand-washing, especially after going into public places or riding public transportation.

The US stock markets tumbled Monday. the Dow Jones Industrial Average fell by more than 450 points, or 1.57 percent, to 28,535.8; the S&P 500 lost 51.84 points, or 1.57 percent, to 3,243.63; and the Nasdaq Composite dropped 175.60 points, or 1.89 percent to 9,139.31.

Investors bought bonds as a safe haven. Airline, cruise and hotel stocks dropped on fears that the virus could limit global travel.

Bloomberg News reported that China's markets will remain closed until Feb 3 after officials extended the Lunar New Year break three days as part of the effort to contain the virus. Trading will resume next Monday in Shanghai and Shenzhen, but authorities advised that most companies shouldn't resume work until at least Feb 9.

Dr Paul Stoffels, Johnson & Johnson's chief scientific officer, said the drugmaker can develop a vaccine in a few months to fight the coronavirus, but that it could take as much as a year to bring it to market.

"We have dozens of scientists working on this, so we're pretty confident we can get something made that will work and stay active for the longer term," he told CNBC. "We'll see in the next few weeks how this goes."