The US said Monday a ban on travelers from China meant to contain the deadly coronavirus outbreak was based on the best science, adding there would be more evacuations of Americans.
It came as the number of confirmed US cases rose to 11, including the second confirmed person-to-person transmission inside the country, this one involving a patient in California.
China has accused the US of spreading "panic" by banning the entry of foreign nationals who had traveled to the country over the past two weeks and announcing quarantine measures for American citizens.
Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the US Centers for Disease Control and Prevention (CDC), defended the "aggressive" measure.
"A couple of weeks ago there were 41 cases in China, this morning the numbers are 17,000 -- that is 17,000 cases with a novel coronavirus that the population does not have immunity to," she said, adding: "We have the opportunity to slow it down before it gets into the United States."
China's foreign ministry spokeswoman Hua Chunying said during a briefing the US had failed to provide "any substantial assistance," a claim that Messonnier denied.
"We at CDC have incredibly strong scientists who have a lot of technical experience in really similar diseases... and our presence on the ground in China will be a help," she said.
"What I've seen is that in situations like this, science should trump everything else."
There are currently 195 Americans including diplomats and their families in quarantine at a military base in California after being evacuated last week from Wuhan, the city at the center of the epidemic, and more such flights were planned, said Messonnier.
"Over the weekend, CDC sent four additional teams to specific Department of Defense locations where those planes will arrive," she said.
Speaking to reporters earlier in Taskhent, Uzbekistan, Secretary of State Mike Pompeo said the timing of the flights was being coordinated with China and would happen "in the next handful of days" while flights from other countries were being considered too.
Messonnier said the 11 US patients had varying degrees of illness -- some "pretty mild," others more severe and requiring oxygen.
Last week the US declared a public health emergency over the pathogen, which has killed more than 360 people and spread globally.
American citizens returning from Hubei province are to be placed in quarantine, while those returning from other parts of China will be put in monitored home isolation if asymptomatic. All will be funneled through 11 major airports.
The ban on foreign nationals who have traveled to China in the past two weeks exempts Hong Kong and Macau.
An illustration of a coronavirus provided by the Centers for Disease Control and Prevention shows its protruding spike-like structures