The United States announced Friday it will provide cluster munitions to Ukraine for the first time as Kyiv's forces face tough fighting in their counteroffensive against invading Russian troops.

The move drew sharp criticism from rights groups due to the danger unexploded bomblets pose, but Washington says it has received assurances from Kyiv that it would minimize risk to civilians, including by not using the munitions in populated areas.

President Joe Biden told CNN that the decision to provide the munitions was "very difficult," but that Ukrainian forces were "running out of ammunition."

"They either have the weapons to stop the Russians now -- keep them from stopping the Ukrainian offensive through these areas -- or they don't. And I think they needed them," he said.

A new military aid package announced Friday included "dual-purpose improved conventional munitions," the Pentagon said in a statement, referring to cluster munitions.

Ukrainian President Volodymyr Zelensky expressed gratitude for the "much-needed" aid, tweeting that "the expansion of Ukraine's defense capabilities will provide new tools for the de-occupation of our land and bringing peace closer."

US National Security Advisor Jake Sullivan sought to justify the decision, saying there was "a massive risk of civilian harm if Russian troops and tanks roll over Ukrainian positions and take more Ukrainian territory and subjugate more Ukrainian civilians."

Kyiv "has provided written assurances that it is going to use these in a very careful way," he said, noting that Ukraine's government "has every incentive to minimize risk to civilians, because it's their citizens."

US Undersecretary of Defense Colin Kahl said Ukraine pledged that it would not use the rounds in civilian-populated areas, and would record where they use them to assist demining efforts after the war.

- 'Keep them in the fight' -

The United States would also not provide cluster munitions with a "dud rate" of more than 2.35 percent, contrasting this with 30-40 percent rate of such weapons used by Russia in Ukraine, Kahl said.

Referring to the Ukraine push, he said that "it's been hard sledding, because the Russians had... six months to dig in. And so those defensive belts that the Russians have put in place in the east and in the south are hard -- they'd be hard for any military to punch through."

"We want to make sure that the Ukrainians have sufficient artillery to keep them in the fight in the context of the current counteroffensive, and because things are going a little slower than some had hoped, there are very high expenditures of artillery."

Rights groups have come out strongly against the United States providing the munitions.

Human Rights Watch said that "transferring these weapons would inevitably cause long-term suffering for civilians and undermine the international opprobrium of their use opposes."

And Amnesty International said Biden's administration "must understand that any decision enabling the broader use of cluster bombs in this war will likely lead to one predictable outcome: the further death of civilians."

"Cluster munitions are an indiscriminate weapon that presents a grave threat to civilian lives, even long after a conflict has ended. Their transfer and use by any country under any circumstances is incompatible with international law," it added.

The United States has spearheaded the push for international support for Ukraine, quickly forging an international coalition to back Kyiv after Russia invaded and coordinating aid from dozens of countries.

Washington has agreed to provide more than $42 billion in military aid to Kyiv since the February 2022 Russian invasion.

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