White House National Security adviser Jake Sullivan said the United States will not leave Ukraine "defenseless" in its war against Russia, speaking to the arrival of cluster munitions in the foreign nation this week.

Sullivan said in a Sunday interview with Chuck Todd on Meet the Press that the U.S. has not stepped away from its moral authority and was never a member of the Convention Against Cluster Munitions — a treaty that prohibits the use, production, transfer, and stockpiling of cluster munitions.

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"Our moral authority and Ukraine's moral authority in this conflict comes from the fact that we are supporting a country under brutal, vicious attack by its neighbor with missiles and bombs raining down on its cities," Sullivan said.

"I would say that we are stepping up to give Ukraine what it needs in order to not be defenseless in the face of a Russian onslaught. We are simply not going to leave Ukraine defenseless," Sullivan added.

Cluster munitions are a specific type of munition that contain tens of submunitions within it, and when it gets near the target, the munition explodes, sending the submunitions over a much more expansive area. Unexploded ordnance can pose a threat to civilians years after a conflict ends.

The U.S. and Ukraine are not members of the Convention of Cluster Munitions, but most NATO members are, which is why many are not following President Joe Biden's lead and sending similar weapons to Ukraine.

It took less than a week for the U.S. to send the controversial munitions to Ukraine. The Defense Department said it had "hundreds of thousands" of cluster munitions available to provide the country.

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Sullivan said Sunday that the Biden administration's current plan is "not to replenish that stockpile" of cluster munitions and will focus on producing other non-cluster ammunition rounds.

"We began that process months ago as we anticipated the need for continuing supply to Ukraine, but it takes time," the national security adviser said. "And that is why we need a bridge from today when we need to ensure that Ukraine has the necessary supply of ammunition to a few months down the road when we believe we can supply enough of the unitary round to meet Ukraine's defense needs."