WASHINGTON (AP) – Cluster munitions provided by the United States have now arrived in Ukraine, the Pentagon said Thursday.

The munitions—which are bombs that open in the air and release scores of smaller bomblets—are seen by the U.S. as a way to get Kyiv critically needed ammunition to help bolster its offensive and push through Russian front lines. U.S. leaders debated the thorny issue for months, before President Joe Biden made the final decision last week.

U.S. leaders have said America will send a version of the munition that has a reduced “dud rate,” meaning fewer of the smaller bomblets fail to explode. The unexploded rounds, which often litter battlefields and populated civilian areas, cause unintended deaths. U.S. officials said Washington will provide thousands of the rounds, but provided no specific numbers.

Lt. Gen. Douglas Sims, the director of operations for the Joint staff, told reporters on Thursday that “cluster munitions have indeed been delivered to Ukraine at this point.” But it was not clear if Ukrainian troops have used them yet.

Mr. Biden described the decision to provide the projectiles as “very difficult,” citing their record of killing civilians. Over 120 countries across the world—but not the U.S., Russia or Ukraine—have signed on to an international convention prohibiting the production of cluster munitions and discouraging their use. Both Moscow and Kyiv have deployed the munitions during the war, and Ukrainian regional officials have regularly accused Russian forces of using them to target civilians.

Ukrainian President Volodymyr Zelenskyy on Wednesday thanked Mr. Biden for the U.S. military aid, and said shipments of controversial cluster munitions would help Ukraine’s fight against Russia. The two met during NATO’s summit in Lithuania, where Western nations made fresh pledges of weapons and ammunition to fight Russia’s invasion.