An explosion near the Jraischan border crossing between Iraq and Kuwait on Monday evening targeted a convoy carrying equipment for U.S. forces, three Iraqi security forces told Reuters.
It was not immediately clear if there were any U.S. troops in the convoy or if anyone had been injured in the explosion, which went off just before 9 p.m. Baghdad time (1800 GMT).
The Iraqi military denied the incident took place.
Kuwait's Armed Forces on Twitter denied the attack on one of the outposts on its northern border with Iraq, affirming that boarders are stable and secure.
Vehicles are regularly loaded with military equipment at the crossing, the sources said, and the cargo is usually loaded or unloaded before entering or exiting Iraq.
Foreign companies are contracted by U.S. forces to provide security in the area, the Iraqi security sources said.
A security source had earlier said that the explosion was caused by an Iraqi Shi'ite Muslim militia targeting a U.S. military base near the crossing by smuggling in an explosive device, and that some staff on the base had been injured. This was later contradicted by other security sources who said a convoy was attacked, not a base.
The U.S. Embassy in Kuwait said it was looking into the matter.
A little known Iraqi Shi'ite militia group by the name of Ashab al-Kahf claimed responsibility for the attack and published a video showing an explosion at a distance. It said it was able to destroy U.S. military equipment and large parts of the crossing.