Two explosions and shooting were heard at a U.S. base in the eastern Afghan city of Jalalabad on Sunday, a Reuters witness said.
The Taliban said they carried out an attack on the compound and inflicted casualties. NATO officials were not immediately available for comment.
Security forces surrounded the area.
"We can confirm there have been multiple explosions in the vicinity of the Jalalabad airfield," the U.S.-led coalition said in a statement.
"Currently, ISAF officials are on the scene gathering facts, and as more information becomes available, we will release it as appropriate."
In February, a suicide car bomber killed nine people at the base at an airport, almost exclusively used by NATO and the U.S. military.
The United States and Afghan government are scrambling to improve security before most NATO combat troops withdraw at the end of 2014.