Cluster munitions are prohibited by more than 100 countries. They typically release large numbers of smaller bomblets that can kill indiscriminately over a wide area and those that fail to explode pose a danger for decades after a conflict ends.

Human rights groups oppose Washington's decision, but the munitions could provide a boost to a Ukrainian counteroffensive to reclaim territory seized by Russian forces since they invaded in February 2022.

U.N. Secretary-General Antonio Guterres is against the continued use of cluster munitions, a U.N. spokesperson said on Friday when asked about the planned U.S. announcement.

U.S. ally Germany said on Friday it opposes sending cluster munitions to Ukraine, German Foreign Minister Annalena Baerbock said. Germany is one of 111 states party to the Convention on Cluster Munitions. The U.S. is not a party to the convention.

Ukraine has asked for cluster munitions to fire against Russian positions with dug-in troops. It has urged members of the U.S. Congress to press President Joe Biden's administration to approve sending them.

"Undoubtedly, the transfer of additional volumes of shells to Ukraine is a very significant contribution to the acceleration of de-occupation procedures," presidential political adviser Mykhailo Podolyak said on Friday.

"Especially if we are talking about cluster ammunition, which is undoubtedly capable of having an extraordinary psycho-emotional impact on already demoralized Russian occupation groups," he said.

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Sending cluster munitions, known as Dual-Purpose Conventional Improved Munitions (DPICM), would ease a drain on standard 155 millimeter shells that Washington has been shipping to Kyiv in massive quantities.

Human Rights Watch has accused both Russian and Ukrainian forces of using cluster munitions that have killed civilians.

A 2009 law bans exports of U.S. cluster munitions with bomblet failure rates higher than 1%, which covers virtually all of the U.S. military stockpile. Biden can waive prohibitions around the munitions as U.S. President Donald Trump did in January 2021 to allow the export of cluster munitions technology to South Korea.

The package was expected to include cluster munitions fired by a 155 millimeter Howitzer canons, said three U.S officials speaking on condition of anonymity. It is also due to include munitions for High Mobility Artillery Rocket Systems (HIMARS), and ground vehicles such as Bradley fighting vehicles and Stryker armored personnel carriers, the officials have said.

The security assistance package would be the 42nd approved by the United States for Ukraine since the invasion, for a total of more than $40 billion.