Some American-made, precision-guided weapons supplied to Ukraine have proved ineffective on the battlefield, their accuracy badly diminished by Russian jamming efforts, according to Ukrainian commanders and a Ukrainian military research project.

The projectiles performed well when first introduced to the battlefield, but lost effectiveness as Russian forces adapted their defenses, two confidential Ukrainian reports found. The problem prompted the Ukrainian military to stop using the weapons, two artillery commanders said.

The reports, first revealed by The Washington Post, focus on the American-made Excalibur, a 155 mm guided artillery shell, and the Ground Launched Small Diameter Bomb (GLSDB). One of the reports was shown to The New York Times by people familiar with the research. The second report was described but not shown to a reporter. The individuals asked not to be identified because the reports contain classified military information.