WASHINGTON — The roadside bomb that disabled an American vehicle in Niger last week was rudimentary and harmed no one, but its location — roughly 70 miles into the country’s interior and minutes from an American Army outpost — was unusual and alarming, military officials said.

The strike was the latest in a string of attacks carried out by Islamic State affiliates using roadside bombs in Niger that have, until now, mostly targeted Nigerien forces.

No extremist group claimed responsibility for the blast, which occurred on the outskirts of the small town of Oullam. The blast’s proximity to American troops has increased concerns about the growing threat of improvised explosive devices, or I.E.D.s, that have long defined American conflicts in Iraq and Afghanistan.

The bomb used on June 8, according to a military official, was activated by a weatherproofed pressure plate and wired to an 81 millimeter mortar projectile, which exploded, causing a main charge of nearly a dozen 60 millimeter mortar rounds to detonate. The explosives were buried roughly three feet underground and positioned at a key choke point on a road that led to a shooting range frequented by American, Canadian and Nigerien troops.