KYIV, Ukraine — Ukraine has begun firing U.S.-provided cluster munitions against Russian forces in southeastern Ukraine in a push to break up well-fortified Russian positions that have slowed Ukraine's summer offensive, according to Ukrainian officials familiar with the matter.

The use of the controversial U.S.-made weapons, which has not been previously reported, follows President Biden's "difficult decision" to order the delivery of the widely banned munitions last week, a move that human rights groups, European allies and some Democrats criticized due to the risk of civilian casualties.

Russian President Vladimir Putin, referring to cluster munitions on Sunday, threatened to retaliate against Ukraine "if they are used against us," although Russia has already used cluster munitions in populated areas of Ukraine at least 24 times since the start of Moscow's invasion on Feb. 24, 2022, according to the United Nations.

Cluster bombs, outlawed in more than 120 countries, explode in the air over a target, releasing dozens to hundreds of smaller bomblets across an area potentially as big as several football fields. Children are particularly vulnerable, as the submunitions can fail to explode until they're picked up, potentially years after a conflict has ended.

But Ukrainian officials have long requested the munitions, which they say will compensate for their significant disadvantage in artillery, weaponry and troop numbers.

Earlier this week, Col. Oleksandr Bakulin, commander of the 57th Brigade, told BBC News that cluster munitions were needed to "inflict maximum damage on enemy infantry," although he acknowledged they would not "solve all our problems on the battlefield."

In addition to front line positions in southeast Ukraine, the cluster munitions are also expected to be used near the Russian-controlled city of Bakhmut, the site of the war's longest and bloodiest battle,

Russia's strongholds in the east and south have been densely mined with antitank and antipersonnel mines and trip wires in areas between three and 10 miles deep. The defenses have been successful in stalling Ukraine's counteroffensive, which began about a month ago.

A Ukrainian official told The Washington Post that the munitions have been fired at Russian positions to break up the trenches slowing down Ukrainian forces seeking to retake territory. The official, like others, spoke on the condition of anonymity to discuss sensitive military operations.

Biden's move to send the munitions followed months of debate within the administration. The president's decision bypassed U.S. law prohibiting the production, use, or transfer of cluster munitions with a "dud rate" of more than 1 percent. The dud rate refers to the share of munitions that remain unexploded.

Biden circumvented the law under a rare provision of the Foreign Assistance Act, which allows the president to provide aid, regardless of arms export restrictions, as long as he determines that doing so is in vital U.S. national security interest.

Defense Secretary Lloyd Austin defended the decision last week, saying the cluster munitions provide a "bridging capability" to keep pressure on the Russians until Western arms production picks up and Kyiv will no longer need the hazardous weapon. Austin said Ukraine committed "in writing" not to use the munitions against population centers and that efforts would be made to try to clean up areas where the weapons are deployed.

"They will record the places that they use them, and they will prioritize demining efforts and we'll help them do that in those places where they have used these," Austin said.

While Russia has used cluster munitions far more extensively, Ukraine has also allegedly deployed these weapons during the war, using its Soviet-era stocks or shells obtained from other countries. A Human Rights Watch report released earlier this month said Ukraine's use of them "caused numerous deaths and serious injuries to civilians" in attacks in the city of Izyum and other locations in 2022. Ukraine has denied using the munitions.

Most of Washington's NATO allies have signed the Convention on Cluster Munitions, a compact that bans their use and production, but the United States, Russia and Ukraine never signed the treaty.

White House national security adviser Jake Sullivan pushed back on the idea that sending the munitions called U.S. moral authority into question.

"Our moral authority and Ukraine's moral authority in this conflict comes from the fact that we are supporting a country under a brutal, vicious attack by its neighbor with missiles and bombs raining down in its cities, killing its civilians, destroying its schools, its churches, its hospitals," Sullivan said. "And the idea that providing Ukraine with a weapon in order for them to be able to defend their homeland, protect their civilians, is somehow a challenge to our moral authority — I find questionable."

Khurshudyan reported from Kramatorsk, Ukraine.

previous coverage

• Pentagon announces new long-term arms deal for Ukraine worth $1.3 billion