Ukraine starts using American-made cluster munitions in its counteroffensive, U.S. officials say.

Ukrainian forces have started using American-made cluster munitions against Russia’s defensive positions, two United States military officials said on Thursday.

The officials, who spoke on the condition of anonymity to discuss operational matters, said the weapons were being used in multiple locations in southeastern Ukraine, but did not elaborate further.

The Washington Post first reported that Ukrainian forces were using the weapons, citing Ukrainian officials. The Post said that Kyiv had begun firing cluster munitions in a push to break through well-established Russian defenses that have slowed Ukraine’s counteroffensive campaign to retake territory.

The Ukrainian authorities did not immediately respond to a request for comment about whether they had started using the weapons.

Hundreds of thousands of the weapons arrived in Ukraine from U.S. military depots in Europe, Pentagon officials said last week. Colin H. Kahl, who recently stepped down as the under secretary of defense for policy, previously said that while “no one capability is a silver bullet,” the cluster munitions would allow Ukraine “to sustain the artillery fight for the foreseeable future.”

President Biden had for months wrestled with the decision over whether to supply the weapons, which have been outlawed by many of America’s closest allies. They scatter tiny bomblets across the battlefield that can cause grievous injuries even decades after the fighting ends when civilians pick up duds.

Russia has used weapons of this type in Ukraine for much of the war. Ukrainian forces have also used them, and President Volodymyr Zelensky of Ukraine had pressed for more in order to flush out the Russians who are dug into trenches and blocking his country’s counteroffensive.