The Pentagon is putting off indefinitely a ban that was due to take effect in 2019 on older models of cluster bombs which are less safe.
The weapons scatter bomblets across a wide area, some of which fail to explode, posing a threat to civilians.
The US military had been hoping to update them with munitions that they say explode at least 99% of the time.
But it now admits that safety technology hasn’t progressed enough to replace existing stockpiles.
“Although the Department seeks to field a new generation of more highly reliable munitions, we cannot risk mission failure or accept the potential of increased military and civilian casualties by forfeiting the best available capabilities,” said a Pentagon policy memo, seen by Reuters news agency.