The German and Dutch intelligence services said Friday that Russia is increasing its use of chemical weapons in Ukraine.

Moscow’s military is employing “a wide range of chemical weapons with increasing frequency and is not hesitating to employ more potent agents,” the spies said in a joint press release published Friday morning.

“We have noticed a growing readiness to employ such weapons. The threat emanating from the Russian chemical weapons program is consequently increasing,” said Dutch military intelligence director Vice Admiral Peter Reesink in a statement.

Dutch Defense Minister Ruben Brekelmans called for tougher sanctions on Moscow in response to the alleged violations, during an interview with Reuters.

He said the intensifying use of chemical weapons was “concerning,” and warned that Russia’s deployment of such weapons is becoming more “normalized and widespread.”

According to the Reuters report, Russian troops have used drones to drop the choking agent into trenches, forcing Ukrainian soldiers out to make them easier targets.

Russia has used chloropicrin, a toxic substance that can cause dyspnea which complicates breathing, the intelligence services said. The chemical weapon was originally deployed during World War I and can be deadly in higher concentrations. At least three Ukrainians have died because of Russian chemical weapons use, according to Ukrainian authorities cited in the joint press release.

Chemical and biological weapons have been prohibited by the Geneva Protocol of 1925 after they were heavily used during brutal trench warfare in World War I. In 1992, the Chemical Weapons Convention expanded these restrictions to the development and stockpiling of chemical weapons.

In May of last year, the U.S. also accused Russia of violating the convention by deploying chemical weapons on the battlefield in Ukraine.