Cluster munitions reach Ukraine a week after Biden’s announcement.

U.S.-provided cluster munitions have arrived in Ukraine, U.S. and Ukrainian officials have said, a week after President Biden announced he had made the “very difficult decision” to provide Kyiv with the widely banned weapon.

Lt. Gen. Douglas A. Sims II, the director of operations for the military’s Joint Staff in Washington, confirmed in a Pentagon briefing on Thursday that the cluster munitions, which break apart midair to release deadly bomblets across a wide area, had been delivered to Ukraine. Brig. Gen. Oleksandr Tarnavsky of the Ukrainian military said in an interview with CNN that the weapons could “radically change” the situation on the battlefield. He said the weapons had not been used as of Thursday.

More than 100 countries — including many U.S. allies — have banned the use of cluster munitions because of the risk they pose to civilians. The weapons often fail to immediately explode, and years or even decades later, they can kill adults and children who stumble upon them. Russia, Ukraine and the United States have not signed an international treaty prohibiting their stockpiling or use.

The Pentagon declined to give details on how many of the weapons were being provided or the timeline of delivery but said the Ukrainian leadership had assured the United States that they would not be used near civilian populations.

Russia reacted to the U.S. decision this week by saying it would respond by itself deploying the weapons in the war. Extensive use of cluster munitions by Moscow’s troops in Ukraine since the beginning of the full-scale invasion is already well documented.

“Russia is constantly using cluster munitions on our territory, fighting exclusively on our land, killing our people, using cluster munitions for many years,” President Volodymyr Zelensky of Ukraine said at this week’s NATO summit in Lithuania. He pledged that his country’s forces would use them “exclusively for military purposes, exclusively for use on the temporarily occupied territories of Ukraine.”

The Biden administration long resisted providing Ukraine with the weapons because of opposition from Western allies — and within his own party — that doing so would hurt the United States’ effort to claim the moral high ground. But last week, Mr. Biden and administration officials said the weapons were necessary, as Ukraine runs low on ammunition and its counteroffensive faces dug-in Russian defenses.

Opponents of the use of cluster munitions argue that the longer-term risk to civilians — which often persists well after hostilities have ended — outweighs short-term strategic benefits.

The White House announcement that it would supply cluster munitions to Ukraine followed assurances from Pentagon officials that the weapons had been improved to minimize the danger to civilians.

The Pentagon said the weapons they would send to Ukraine had a failure rate of 2.35 percent or less, far better than the usual rate. But the Pentagon’s own statements indicate that the cluster munitions in question contain older grenades known to have a failure rate of 14 percent or higher.

On Thursday night, a proposal from some conservative Republicans — who had aligned themselves with Democrats — that would have barred the administration from sending cluster munitions to Ukraine failed in the House of Representatives.