“The coming days and weeks are likely to bring more cases including the possibility of person-to-person spread,” he said. “Our goal is to contain this virus and prevent sustained spread of the virus in our country.”

[A plane carrying Americans is leaving from Wuhan. If you know anyone on board, or anyone trying to leave Wuhan, we would like to hear from you for a coming article. Please contact Miriam Jordan at miriam.jordan@nytimes.com to share your story.]

Officials also announced that after repeated offers of assistance, Chinese authorities agreed on Tuesday to allow in teams of international experts, coordinated by the World Health Organization, to help with research and containment.

“Americans should know this is a potentially very serious public health threat, but at this point Americans should not worry for their own safety,” said Alex M. Azar, secretary of health and human services, at the briefing.

Scientists race to make a coronavirus vaccine.

Scientists in the United States, Australia and at least three companies are working on vaccine candidates to stop the spread of the coronavirus.

Government scientists as well as those working at Johnson & Johnson, Moderna Therapeutics and Inovio Pharmaceuticals are all working quickly to develop a vaccine.

The hunt began Jan. 10, when Chinese scientists posted the genetic makeup of the virus on a public database. The next morning, researchers at the National Institutes of Health’s Vaccine Research Center in Maryland went to work. Within hours, they had pinpointed the letters of the genetic code that could be used to make a vaccine.