More than 100 people in 26 states are being monitored for the new coronavirus that has killed 81 people in China, a U.S. health official said Monday.

The news comes as the CDC on Monday raised its travel alert level for China to level 3, meaning Americans should avoid nonessential travel to the entire nation.

Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said 110 people are "under investigation" for the virus but added that human-to-human transmission of the virus has not been documented in the U.S.

"This is a rapidly changing situation, both here and abroad," Messonnier said. "However, the immediate risk to the general U.S. public is low at this time."

Still, she said a program for screening travelers entering the U.S. from Wuhan could be expanded in the coming days to include other parts of China. President Donald Trump tweeted Monday that U.S. officials are in "very close communication" with China and offered aid to President Xi Jinping as his nation grapples with the coronavirus.

China's confirmed cases have ballooned to more than 2,800 since the coronavirus was discovered last month. The epicenter of the outbreak is Wuhan, a city of 11 million people, but more than 40 cases have been confirmed in a dozen other countries, including five in the U.S.

"All US cases traveled from Wuhan," the Centers for Disease Control and Prevention said in a statement. "More cases may be identified. However, risk to US general public is still considered low."

In the U.S., two cases have been confirmed in California, one in Arizona, one in Illinois and one in Washington state. Dozens of cities and states continue to screen patients whose symptoms are consistent with the virus.

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

Halting the outbreak remains a challenge, however. China’s National Health Commission says it appears the virus is infectious before symptoms show, an alarming characteristic that could complicate containment efforts.

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That could make this outbreak more ominous than severe acute respiratory syndrome, SARS, a virus that was not contagious during incubation. SARS killed more than 600 people across mainland China and Hong Kong along with more than 100 other people around the world in 2002-2003.

Ogbonnaya Omenka, an assistant professor and public health specialist at Butler University's College of Pharmacy and Health Sciences, says the revelation makes the coronavirus "more problematic and more difficult to contain."

"It poses additional challenges," Omenka told USA TODAY. "Now we’re looking at both the identified cases and the traced contacts needing to be isolated to ensure the contacts do not keep spreading the virus unwittingly."

In China, the government extended its Lunar New Year holiday period by three days, until Sunday, to ease crowds as tens of millions of Chinese return home from visiting family and tourist sites. At least 17 Chinese cities have imposed lockdowns affecting 50 million people.

Chinese Premier Li Keqiang arrived Monday in Wuhan to take charge of the effort to curb its spread, the South China Morning Post reported.

“You are trying every means to save lives,” Li told medical staff at Jinyintan hospital, one of the designated institutions for coronavirus patients. “When you are putting your efforts to save lives, you have to protect yourselves, too.”