The Biden administration announced on Wednesday that travelers from China, Hong Kong and Macau must present negative Covid-19 tests before entering the United States, a move that it says is intended to slow the spread of the coronavirus. The requirement will take effect on Jan. 5.

The announcement, by the Centers for Disease Control and Prevention, came amid growing concern over a surge of cases in China and the country’s lack of transparency about the outbreak there.

C.D.C. officials said the requirement for testing will apply to air passengers regardless of their nationality and vaccination status. It will also apply to travelers from China who enter the United States through a third country, and to those who connect through the United States to other destinations.

China’s Covid outbreak has been worsening in recent days, with local governments reporting hundreds of thousands of infections a day. Videos obtained by The New York Times show sick patients crowding hospital hallways. But the situation is difficult to track in real time because China does not release reliable Covid data.

This is a developing story. Check back for updates.