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-millimeter guided artillery shell, and the Ground Launched Small Diameter Bomb or 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.

Every war is a laboratory of sorts for weapons systems, and Ukraine has provided a testing ground for weapons that have not previously been used against such a sophisticated, high-tech enemy as Russia. The performance of U.S. weapons and Russian weapons, defensive as well as offensive, is closely monitored by the Pentagon and NATO — as well as Russia and China — with significant implications for future weaponry.

More immediately, Ukrainian commanders say, some of the Western weapons supplied to them have failed them at the cost of lives.

Russia has deployed electronic warfare systems around static targets such as headquarters and command centers that might be targets of Ukrainian precision weapons. The systems blast out so much interference that they drown out the GPS signal that guides the Excalibur’s targeting software, said Thomas Withington, an associate fellow at the London-based Royal United Services Institute and a specialist in electronic warfare.