The United States is expected to announce that it will provide Ukraine with cluster munitions, a senior Biden administration official said. Kyiv has been pushing for the controversial and widely banned type of weapon but Washington has resisted because of its potential to cause indiscriminate harm to civilians.

Ukraine has said the weapons would help in its counteroffensive against Russian troops by allowing its forces to effectively target entrenched Russian positions and to overcome its disadvantage in manpower and artillery.

After months of demurring, citing concerns about the weapons’ use and saying they were not necessary, U.S. officials have recently signaled a shift. Laura Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, told U.S. lawmakers late last month that the Pentagon had determined that cluster munitions would be useful for Ukraine, “especially against dug-in Russian positions on the battlefield.”

The expected U.S. decision was first reported by National Public Radio and confirmed on Wednesday night by the administration official, speaking on condition of anonymity in order to disclose internal policy discussions.

Here is what to know about the weapons.

Cluster munitions, first used during World War II, are a class of weapons including rockets, bombs, missiles and artillery projectiles that break apart midair and scatter a number of lightweight bomblets over a large area.

Cluster munitions’ bomblets are designed to explode or ignite upon hitting the ground, but historically the failure rate is high, with sometimes lasting and devastating consequences for civilians. According to humanitarian groups, a fifth or more of bomblets can linger, potentially to detonate when disturbed or handled years later.

Since World War II, cluster munitions have killed an estimated 56,500 to 86,500 civilians. They have also killed and wounded scores of American service members. Civilians, including children in Syria, Yemen, Afghanistan, Lebanon, the Balkans and Laos, continue to suffer from incidents involving remnants of cluster munitions.

While the deployment of cluster munition isn’t in and of itself a war crime, their use against civilians can be, because they kill so indiscriminately with long-lasting effects.

Because of those risks, more than 100 countries — though not the United States, Russia or Ukraine — have signed a 2008 treaty known as the Convention on Cluster Munitions, promising not to make, use, transfer or stockpile them. Since the adoption of the convention, 99 percent of global stockpiles have been destroyed, according to the Cluster Munition Coalition.

Ukraine has said it would deploy the weapons judiciously, given that it is fighting on its own land, and that many frontline areas are already widely affected by land mines.

Russia has extensively used cluster munitions in Ukraine since the beginning of the invasion in February 2022. Ukraine has also used them in efforts to retake Russian-occupied territories, according to human rights monitors, the United Nations, and reports from The New York Times. The Cluster Munition Coalition said in its annual report last summer that cluster munitions had killed at least 689 people in just the first six months of fighting.

While the exact number of the weapons used in the conflict is difficult to know, hundreds have been documented and reported in Ukraine, mostly in populated areas, the group Human Rights Watch said in a May 2023 report. The attack with the highest known casualties was an April 2022 strike with a missile equipped with a cluster munition at a crowded train station in Kramatorsk, which killed dozens and injured more than 100 others, according to the group.

“Transferring cluster munitions disregards the substantial danger they pose to civilians and undermines the global effort to ban them,” Mary Wareham, the group’s arms advocacy director, said in a statement on Thursday.

Most members of NATO, the Western military alliance that has been staunch in its support for Ukraine, have signed on to the international ban. Ms. Cooper, the deputy assistant secretary of defense, said “concerns about allied unity” was one of the reasons holding the United States back from providing the weapons to Ukraine.

Ahead of Ukraine’s long-anticipated counteroffensive, Russian troops have had months to prepare lines of defense against the coming assault, with miles of trenches, tank traps and mines. The cluster munitions could help the Ukrainian forces, which are outnumbered by the Russian military, overcome those defenses.

In February, Oleksandr Kubrakov, Ukraine’s deputy prime minister for restoration, said speedy provision of arms from allies would be critical to Kyiv’s advance in the counteroffensive against Russia, and that it should be Ukraine’s choice to deploy the weapons on its soil.

“It’s our territory. I understand how it’s complicated with all these conventions, but we can use to resist them on our territory,” he said in a town hall at the Munich Security Conference. “Our allies, the U.S., many other countries, they have millions of rounds of such type. Again, we will wait, wait, wait, and suddenly one day, probably, we will receive such type of munitions.”

Eric Schmitt contributed reporting.