China's health authorities on Tuesday reported 21 additional cases of novel coronavirus infection on the mainland, all but one from people arriving from abroad.

The National Health Commission also reported 13 more deaths, 12 of them in hard-hit Hubei Province in central China, taking the total number of deaths on the mainland to 3,226.

The only locally transmitted case reported over a one-day period to the end of Monday occurred in Wuhan, Hubei's provincial capital and the epicenter of the country's virus outbreak, with the remaining 20 detected in passengers arriving from overseas in cities such as Beijing and Shanghai.

The tally of confirmed cases for mainland China now stands at 80,881. The number of imported cases is 143.

The number of new infections found in travelers from abroad has outnumbered those newly found infected inside China in recent days, prompting the Chinese government to clamp down on imported cases.

At least 16 people who returned to China from overseas are currently being investigated for allegedly attempting to conceal their infection, a Beijing municipal government official said Monday.

Beijing police have said a 37-year-old woman is under investigation after she took a fever medicine and boarded a flight from the United States. She tested positive for the virus after her arrival in Beijing last week, they said.

Of the 31 Chinese citizens who returned from countries including Italy and Spain and have since been confirmed to be infected with the virus, 27 had exhibited symptoms before arrival, according to the municipal official.

Following several days of single-digit new infections reported in Wuhan, local authorities said in an online notice on Tuesday that the city, a transport and manufacturing hub, will quarantine all overseas returnees at designated facilities from the same day.

Beijing implemented similar quarantine measures just a day earlier. Both cities require travelers to bear all isolation costs.

The Foreign Ministry on Tuesday also discouraged its citizens from traveling to 15 high-risk countries, namely Austria, Belgium, Britain, Denmark, France, Germany, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland, the United States, Iran and South Korea.

Japan, which has been singled out by several local authorities including Beijing as a high-risk country, was not included in the travel warning.

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