Secretary of State Antony Blinken on Tuesday defended the Biden administration’s decision to send cluster munitions to Ukraine as it wages a counteroffensive against Russian forces, saying it was a decision of last resort in the face of supply constraints.

“The stockpiles around the world and in Ukraine of the unitary munitions, not the cluster munitions, were running low. They’re about to be depleted,” Blinken said in an interview with NBC’s Andrea Mitchell. “The hard but necessary choice to give them the cluster munitions amounted to this: If we didn’t do it, we don’t do it — then they will run out of ammunition. If they run out of ammunition, they will be defenseless.”

Blinken’s comments come amid a summit of NATO members in Vilnius, Lithuania, where members have considered the entry of two new countries, Finland and Sweden, into the alliance, as well as the war in Ukraine. The comments also come on the heels of last week’s announcement from the Biden administration that it would send cluster munitions to Ukraine. Cluster munitions, which are officially termed dual-purpose improved conventional munitions, are designed to take out multiple military targets by scattering large numbers of explosive “bomblets” over a wide area. Both sides in the ongoing war in Ukraine have used cluster munitions, with Ukrainian cluster munitions killing eight civilians in Izium last year, according to Human Rights Watch.

However, cluster munitions have historically left behind unexploded ordinances that can endanger civilians well after conflicts end. More than 100 countries, including most members of NATO, have signed onto the 2010 Convention on Cluster Munitions, which prohibits their use, transfer, production and stockpiling; the United States, Russia and Ukraine are not among the signatories.

U.S. allies, as well as human rights groups, have criticized the decision. The Biden administration has also faced outrage from some key Democrats on Capitol Hill, who broke with the president publicly last week over the decision.

Ukraine, as well as Republicans on Capitol Hill, have celebrated the U.S. decision to send the munitions. Speaking to reporters in Vilnius, Ukraine’s defense minister Oleksiy Reznikov said Tuesday that he hoped that the shipment of cluster munitions would be the “next game changer as weaponry or ammunition for liberation of our temporarily occupied territories.”

Blinken has denied that the U.S. was losing credibility with its allies over the decision. “Every ally I’ve talked to has said they understand why we’re doing this when we’re doing it,” Blinken said Tuesday.