Ukrainian troops have started firing the cluster munitions provided by the US as part of their counteroffensive against Russia, according to two US officials and another person briefed on the matter.

The US is still waiting for updates from Ukrainian forces about how effective the munitions have been on the battlefield, one of the officials said.

National Security Council coordinator for strategic communications John Kirby confirmed later on Thursday that Ukrainians forces have begun using the munitions.

“They are using them appropriately. They’re using them effectively and they are actually having an impact on Russia’s defensive formations and Russia’s defensive maneuvering. I think I can leave it at that,” Kirby told reporters.

The US announced on July 8 that it would be sending the controversial munitions, and they were delivered to Ukrainian forces about a week later, as CNN first reported.

Brig. Gen. Oleksandr Tarnavsky told CNN at an interview in central Ukraine last week that the munitions “can radically change (the battlefield).”

“The enemy also understands that with getting this ammunition, we will have an advantage,” Tarnavsky said.

Cluster munitions scatter “bomblets” across large areas, which would allow Ukrainian forces to target larger concentrations of Russian forces and equipment with fewer rounds of ammunition.

But the bomblets can also fail to explode on impact, and can pose a long-term risk to anyone who encounters them, similar to landmines. The UK, France, Germany and other key US allies have outlawed the munitions under the Convention on Cluster Munitions, but the US and Ukraine are not signatories to the ban.

The US sent the M864 and M483A1 models of cluster munitions, CNN has reported, which the administration said were tested in recent years to ensure they had a lower than 2.35% dud rate. The dud rate refers to the percentage of bomblets that fail to explode and pose a risk to civilians.

The US decided to send the cluster munitions primarily to help alleviate a potential shortage of ammunition on the frontlines. It is not clear whether the heavy amount of artillery ammunition Ukrainians forces have been expending day-to-day would have been sustainable long-term without the cluster munitions, officials and military analysts said.

CNN reported earlier this week that the US and Europe are struggling to provide Ukraine with the large amount of ammunition it will need for a prolonged counteroffensive against Russia, and western countries are racing to ramp up production to avoid shortages on the battlefield that could hinder Ukraine’s progress.

US Secretary of Defense Lloyd Austin told CNN’s Wolf Blitzer last week that the Ukrainians pledged in writing to only use the cluster munitions in “appropriate places” and not in populated areas.

Russian President Vladimir Putin said earlier this week that Russia has a stockpile of cluster munitions and will consider using them against Ukraine “if they are used against us.” But Russia has already used the munitions several times in Ukraine, CNN has previously reported, including in densely populated areas.

In March, the United Nations said it had compiled credible reports that Russian forces had used cluster munitions in populated areas at least 24 times. A CNN investigation last year found that the Kremlin fired 11 cluster rockets at Kharkiv, Ukraine’s second-largest city, during the war’s opening days.

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