Cluster munitions are a question for individual countries, NATO’s secretary general says.

American allies reacted with caution on Friday to reports that the Biden administration said it would provide Ukraine with cluster munitions, widely banned weapons that often cause grievous injury to civilians, especially children.

While not criticizing the United States or opposing the move, Germany and France said they would not follow suit, pointing to an international treaty they have signed that bans the use, stockpiling or transfer of such weapons. The United States, Russia and Ukraine have not signed the treaty, known as the Convention on Cluster Munitions.

“Germany has also signed the convention; for us this is not an option,” Germany’s defense minister, Boris Pistorius, told reporters in Bern, Switzerland.

The French Foreign Ministry also referred to the treaty, known as the Oslo Convention, saying that France “has pledged not to produce or use cluster munitions, and to discourage their use.” But a spokeswoman for the ministry noted in response to a reporter’s question that neither the United States nor Ukraine were bound by the treaty.

“We understand the decision that the United States have reached to help Ukraine exercise its self-defense against Russia’s illegal aggression,” she said.

President Biden’s approval for supplying Ukraine with the weapons, which Kyiv has long sought,sharply separates him from many of the United States’ closest allies, and complicates allies’ efforts to demonstrate unity at a NATO summit next week in Lithuania.

While top U.S. national security officials have had reservations about providing the weapons, they think they have little choice but to send them to Ukraine, which risks running out of the conventional artillery rounds it needs to fight Russia, according to people familiar with the discussions.

On Friday, Jake Sullivan, the president’s national security adviser, said that Russia had been using cluster munitions since the beginning of the war, and that Ukraine “would not be using these munitions in some foreign land.”

“This is their country they’re defending,” Mr. Sullivan said. “These are their citizens they’re protecting, and they are motivated to use any weapon system they have in a way that minimizes risks to those citizens.”

The NATO secretary general, Jens Stoltenberg, said on Friday that the military alliance did not have a formal position on using cluster munitions in battle, dodging a question on whether he believed it was wise for the United States to provide the widely banned weapons to Ukraine.

“It is for individual allies to make decisions on the delivery of weapons and military supplies to Ukraine,” Mr. Stoltenberg told journalists at NATO headquarters in Brussels. “So this will be for governments to decide — not for NATO as an alliance.”

Russia’s ambassador to Belarus, Boris Gryzlov, called the provision of cluster munitions to Ukraine a “move of desperation.”

“The ‘hawks’ in the West have realized that the much-advertised counteroffensive of the Ukrainian armed forces did not go according to plan, so they are trying at all costs to give at least some impetus to it,” he told the TASS news agency.

Cluster munitions disperse tiny bomblets that sometimes fail to explode on hitting the ground, only to detonate years later when disturbed by civilians. But officials have said the Biden administration now believes the munitions are the best way to kill Russians who are dug into trenches and blocking Ukraine’s counteroffensive to retake territory. One American official said on Thursday that it was now clear that the weapons were “100 percent necessary” to meet battlefield needs.

Mr. Stoltenberg said both Russia and Ukraine were already using cluster munitions. The New York Times has documented Russia’s extensive use of cluster munitions in Ukraine since the start of its full-scale 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 Times.

“Russia uses cluster missions in their brutal war of aggression, to invade another country, while Ukraine is using it to defend itself,” Mr. Stoltenberg said.