"We're concerned by the recent clashes at the Torkham crossing point between Afghanistan and Pakistan. We have gotten reports that casualties have been taken on both sides," State Department Spokesman John Kirby told reporters.
"We're also concerned about reports of the presence of heavy weapons at the border," he said.
Pakistan and Afghan forces exchanged fire on Sunday over construction on the Pakistani side of the border.
Torkham is the most frequently used crossing point at Pak-Afghan; most of the terrorists have been found using this gate for entry.
Good relations between the two countries are key to the stability to the region, he said in response to a question.