The government’s top public health agency is raising the importance of wearing face masks on planes, trains and buses, although the Trump administration has resisted making masks mandatory for travelers.

The Centers for Disease Control and Prevention issued new interim guidelines for travelers, including a “strong recommendation” to wear face coverings to slow the spread of COVID-19.

“Transmission of the virus through travel has led to — and continues to lead to — interstate and international spread of the virus,” the CDC said in a statement. “Local transmission can grow quickly into interstate and international transmission when infected persons travel on public conveyances without wearing a mask and with others who are not wearing masks.”

The CDC said its advice on masks should be followed by passengers and workers on planes, ferries, trains, subways, buses, taxis and ride-sharing vehicles, including in airports and at subway and bus stations.

The new guidance issued Monday includes more specific advice for travelers than CDC has previously given. The agency said it was offering the new recommendations to support state and local health officials and transportation operators who have imposed their own mask rules.

Travelers are often in close contact with others people, sometimes for several hours, raising the risk of contracting or spreading the virus that causes COVID-19. Masks are necessary because travelers might not be able to keep six feet apart from others on planes and buses, CDC said.

All leading U.S. airlines require passengers other than small children to wear masks during flights, but enforcement can be spotty. The Federal Aviation Administration has declined to require masks, passing over requests by airline labor groups and some Democratic lawmakers. Instead, the Transportation Department and other agencies issued health guidelines in July that left mask requirements and enforcement to individual airlines.