The Trump administration on Friday dramatically escalated its response to the fast-spreading coronavirus epidemic by announcing quarantines and major travel restrictions that officials said were meant to limit contagion.

The measures, which could affect thousands of people around the world, represented a marked expansion of the federal government’s response after initially downplaying potential risks.

The White House declared a “public health emergency” and — beginning on Sunday at 5 p.m. — will bar non-U.S. citizens who recently visited China from entering the United States, subject to a few exemptions. Shortly after the White House announced the new restrictions and said there were six confirmed U.S. cases, a seventh case was confirmed in Santa Clara County, Calif.

Health and Human Services Secretary Alex Azar also said the Trump administration would quarantine any Americans who had visited China’s Hubei province, where the disease originated, within the past 14 days. The government will also require screening and self-quarantines for all other Americans who recently visited any other parts of China. Officials said the self-quarantine rules would require individuals to stay in their homes for a certain length of time, monitor themselves for certain symptoms such as coughing, and check their temperatures and report them to local health officials.

The new travel and quarantine measures, which appeared to be unprecedented on such a scale, were part of a rapidly evolving international response to the growing health scare. For the first time Friday, U.S. officials acknowledged that screening tests for coronavirus are not always accurate and that people with no symptoms can transmit the virus. New cases were reported Friday in Britain and Russia, and Canada announced its fourth case later in the day.

“This is a serious health situation in China, but I want to emphasize that the risk to the American public currently is low,” said Robert Redfield, director of the Centers for Disease Control and Prevention. “Our goal is to do all we can do to keep it that way.”

“Right now, there’s a lot of unknowns,” he added later.

White House officials said the new restrictions would be temporary but did not say when they might be lifted. Meanwhile, major U.S. airlines that travel to China — American, United and Delta — announced they were suspending their flights to the country, in some cases until late March. The duration of these cancellations showed how governments and businesses are now preparing for much longer-term interruptions than initially thought, raising the possibility that there could be a sizable impact on the global economy.

Many Chinese businesses remain closed, and China is a top trading partner of a number of countries throughout the world.

These announcements helped send the U.S. stock market sharply lower, with the Dow Jones industrial average falling Friday by a dramatic 600 points, or 2 percent.

At a hastily arranged afternoon press briefing, White House officials stressed that they believed there was little risk to the U.S. public from the coronavirus and that the measures were an attempt to keep it that way. But their comments also revealed how rapidly the process is evolving.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said that the process for testing people for the new strain of coronavirus had proved very imprecise so far and that this is one of the reasons for the aggressive U.S. response. He said there was evidence that people who initially tested negative for the virus actually had contracted it. Officials also believe that at least one person, in Germany, has obtained the virus from someone who didn’t exhibit any symptoms. These developments have caused the U.S. government to markedly escalate its response.

“We still have a low risk to the American public, but we want to keep it at a low risk,” Fauci said, adding that there “are so many unknowns here.”

Ken Cuccinelli, acting director of Citizenship and Immigration Services, said American travelers coming from China would be funneled through seven major airports to ensure they can be screened: New York’s John F. Kennedy International Airport, Chicago O’Hare International Airport, San Francisco International Airport, Seattle-Tacoma International Airport, Hartsfield-Jackson Atlanta International Airport, Daniel K. Inouye International Airport in Honolulu and Los Angeles International Airport.

The mandatory quarantines — an extremely rare step — will apply to U.S. citizens who have been in Hubei province in the past two weeks. All of the new steps will take effect at 5 p.m. Eastern time on Sunday. While many foreign nationals who visited China within 14 days will be banned from entering the United States, officials said those who are family members of U.S. citizens will be allowed to enter subject to new screening guidelines.

Not long before the new policies were announced, U.S. health officials on Friday extended a quarantine for 195 evacuees who arrived from Wuhan this week at a military base in California. Officials had originally said they would be held for 72 hours, but now they will be held up to 14 days since the time they left China.

The coronavirus emerged in China in early December. The virus so far has killed more than 210 people, all of whom have been in China, and infected more than 11,000 people in at least 24 countries. There have been more than 100 cases reported outside China. At least 12 people who have contracted the virus have not recently traveled to China and were infected from other people.

The number of those infected has surpassed the SARS 2003 outbreak, but the new coronavirus appears to be less deadly. Studies have estimated that each infected person spreads the virus to two to three other people. There is no vaccine or treatment for the coronavirus, although Fauci that said researchers have begun developing a vaccine and that a safety study to test it in people could begin in as little as three months.

Late Thursday, the State Department issued a “do not travel” advisory for China, the department’s highest warning level, which previously had been in place only for Hubei province.

U.S. officials are also working to evacuate more Americans from Hubei province beginning next week. The State Department is attempting to stage additional flights for U.S. citizens to return from Wuhan, a State Department spokesperson said, adding that “as space is available, seating will be offered to U.S. citizens on a reimbursable basis.”

Public health experts praised the administration’s decision to declare a public health emergency, which some said was overdue. The emergency declaration gives the government more resources and flexibility to respond to the outbreak. But some experts said that several of the administration’s other actions on Friday were extreme.

“These are extraordinary measures to be implemented at this point,” Ali Khan, former director of the Office of Public Health Preparedness and Response at the CDC, said of restricting foreign nationals. “We’re a global economy, people move, and if you have excellent public health and you are able to combine port-of-entry screening with follow-up of people, there should be no reason to put undue burdens on people around travel.”

President Trump so far has remained uncharacteristically muted on the coronavirus and praised China’s extraordinary response to the growing outbreak. On Wednesday, he tweeted photos of his Situation Room briefing and said his administration was working closely with China to contain the outbreak.

But even before U.S. officials announced such a drastic escalation in travel restrictions on Friday, China’s Foreign Ministry criticized the United States for making “unfriendly comments” during the coronavirus outbreak and said a U.S. warning against travel to China went against recommendations made by the World Health Organization to not restrict travel or trade. “In disregard of WHO recommendation against travel restrictions, the US went the opposite way,” the ministry’s spokesman said in English-language messages on Twitter on Friday. “Where is its empathy?”

Even with the Senate in the midst of Trump’s impeachment trial, a number of GOP lawmakers have begun pushing the administration on its response to the virus. The steps the administration announced did not go as far as some wanted.

Sen. Ted Cruz (R-Tex.) called Friday for the administration to issue an outright ban on travel to and from China.

“Given the severity of the coronavirus in China and its rapid spread across the region, as well as the mounting public fear, it’s imperative that this disease is contained,” Cruz said. “The administration should take every precaution in preventing additional cases from arriving in the United States, including temporarily halting flights to and from China and implementing additional screening at all U.S. airports.”

Before Friday, Trump had shown unusual restraint in his assessment of the spreading epidemic.

During the 2014 Ebola outbreak, before Trump announced a run for president, he heavily criticized the Obama administration’s response. “The U.S. cannot allow EBOLA infected people back. People that go to far away places to help out are great- but must suffer the consequences!” Trump wrote on Twitter at the time.

“STOP THE FLIGHTS!” he wrote in another tweet. “NO VISAS FROM EBOLA STRICKEN COUNTRIES.” And in still another tweet, he falsely claimed that the virus spread more easily than CDC scientists said.

Jeff Stein contributed to this report