The US is set to require all inbound air travellers produce a negative Covid-19 test taken just one day before departure.
It is a response to fears over the Omicron variant.
Under current requirements, international air travellers present a negative test taken within three days of their departure.
All foreign travellers must be vaccinated to enter the US.
The tightened testing rules are now being considered.
"CDC is working to modify the current Global Testing Order for travel as we learn more about the Omicron. A revised order would shorten the timeline for required testing for all international air travellers to one day before departure," the CDC said.
"This strengthens already robust protocols in place for international travel, including requirements for foreign travellers to be fully vaccinated."
The Omicron variant had prompted officials to look at ways to strengthen health surveillance at the border, said Natalie Quillian, the Covid response deputy coordinator.
"We’re constantly looking at what can we do to make that travel system even stronger. For example, right now we’re assessing all of our tests to make sure they’re effective in picking up this variant."
"If we found that a test was not effective at picking up the variant, we would remove that from the list that is accepted to enter the country."
