But the new orders from Resolute Support could stretch that quarantine period for far longer, as the United States-led NATO command puts measures in place to try to slow the spread of the virus into Afghanistan.

One allied official said that even in a worst-case scenario, the readiness of their forces should be only marginally affected. The pause in rotations will make sure personnel coming into Afghanistan are virus-free and minimize any risk that troops leaving would infect others in their home countries.

Allied countries also plan to develop new protective measures, like isolating troops ahead of deployments. In addition, the official said, NATO nations are looking to send more medical staff to Afghanistan to help protect and treat troops deployed there.

One Defense Department official said that the new measures were also intended to stop the spread of the virus via troops coming into Afghanistan from Europe. The fear, another official said, is having virus cases overwhelm a military medical system that is meant to support the war effort. The officials spoke on the condition of anonymity to discuss plans that have not yet been made public.

There are other reasons to hold off on any rotations of European troops in and out of Afghanistan. With other European countries expected to follow France’s domestic military deployments to try to enforce social-distancing rules at home, NATO countries could quickly discover they are running out of available troops.

American officials are particularly worried about the potential impact of the coronavirus on Afghanistan, and deeply concerned that the Afghan security forces, the troops they have trained to protect the country, will end up spreading it there.

In a statement Thursday, General Miller said his forces had reduced “in-person” meetings with Afghan forces and had adjusted their positions on bases to “create physical distance.”