British Defense Secretary Ben Wallace says NATO nations are “struggling to find a way” to maintain Ukraine’s ammunition supply amid the full-scale invasion of Russia continues after 500 days.

“Huge amounts of ammunition are fired and used,” he told a panel at the NATO summit in Vilnius, Lithuania.

Wallace told CNN on Wednesday that because of the shortage, he understands why the United States chose to supply Ukraine with the controversial cluster bombs in an effort to fill the void. But he said that since the UK has passed a ban on such munitions, he cannot promote their use in any way.

“On the one hand, I understand the military necessity the Ukrainians faced,” he said. “And the United States is not a signatory to this treaty, so it gives them the freedom to do what they think is right.”

“They just find ways around the challenge,” Wallace said. But, he added, “we signed the treaty, we feel that we cannot defend the use (of cluster bombs). We cannot offer them. We cannot promote them. We cannot support them. We can’t help them, and that’s what limits them.”

US President Joe Biden told CNN last week that the United States was “running out of supplies” of munitions it could send to Ukraine, which is why Biden decided to send the cluster bombs. The supply of cluster munitions is temporary, National Security Advisor Jake Sullivan said last week, until the United States can increase its production of unitary munitions.

Wallace said the broader issue of a stockpile issue is a serious concern due to the amount of supplies Ukraine needs to carry out its counteroffensive.

“The supply of material from Ukraine has been huge, vast. I think they have prepared 12 brigades for this offensive, two brigades are bigger than most people’s armies, right? 12 brigades actually armored prepared for this counteroffensive, mainly with free equipment and donations,” he said.

“We have all had to fight to optimize our supply chains, some of which have been shut down,” he added.

Wallace also said a specific big challenge they see with Ukraine is its ability to close runways used by Russian planes. The UK-supplied Storm Shadow long-range missile “is a deep strike weapon, but it is not designed to destroy airstrips”, he said. The UK is therefore trying to find ways to help Ukrainian troops improve this capability.