The United Kingdom and Spain warned against President Biden’s controversial decision to send cluster munitions to Ukraine ahead of the NATO summit in Lithuania this week. 

"The U.K. is signatory to a convention which prohibits the production or use of cluster munitions and discourages their use," British Prime Minister Rishi Sunak said in a video Saturday published by Sky News. "We will continue to do our part to support Ukraine against Russia’s illegal and unprovoked invasion, but we’ve done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine."

"Russia’s act of barbarism is causing untold suffering to millions of people," Sunak added. "It’s right that we collectively stand up to it. And I’ll be heading off the NATO summit next week in Vilnius, where we will be discussing exactly this with our allies, how we can strengthen our support for Ukraine." 

US TO PROVIDE CLUSTER MUNITIONS TO UKRAINE, $800 MILLION MILITARY AID PACKAGE, OFFICIALS SAY

Rishi Sunak arrives for magazine party in London

Prime Minister Rishi Sunak, in central London on Wednesday July 5, 2023, has warned against sending cluster munitions to Ukraine. (Lucy North/PA Images via Getty Images)

"Spain, based on the firm commitment it has with Ukraine, also has a firm commitment that certain weapons and bombs cannot be delivered under any circumstances," Spanish Defense Minister Margarita Robles told reporters during a rally in Madrid ahead of the July 23 national election, according to Reuters. "No to cluster bombs and yes to the legitimate defense of Ukraine, which we understand should not be carried out with cluster bombs."

The latest test of NATO solidarity came Friday with what Biden said was a "difficult decision" to provide cluster munitions to Ukraine. 

More than two-thirds of alliance members have banned the weapon because it has a track record for causing many civilian casualties. The U.S., Ukraine and Russia are not among the more than 120 countries supporting the 15-year-old Convention on Cluster Munitions (CCM) prohibiting all use, production, transfer and stockpiling of cluster munitions out of humanitarian concern. The U.K. and Spain, by contrast, are among the nations that signed the convention.

Biden walks to board Air Force One to depart for the UK

President Biden speaks with Col. W. Chris McDonald as he walks to board Air Force One at Dover Air Force Base in Delaware, on July 9, 2023. Biden left Sunday for Britain to meet with King Charles III before continuing to Vilnius for a NATO summit, then a final stop in new NATO member Finland. (Andrew Caballero-Reynolds/AFP via Getty Images)

RFK JR. BLASTS BIDEN FOR SENDING CLUSTER BOMBS TO UKRAINE, A YEAR AFTER PSAKI SAID IT COULD 'BE A WAR CRIME'

Reacting to the U.S. announcement, Ukrainian Defense Minister Oleksii Reznikov tweeted five key principles Kyiv agreed upon regarding the use of cluster munitions, including to use the bombs only "for the de-occupation of our internationally recognized territories," to keep them out of urban areas, keep strict record of the use of those weapons in local zones, to later prioritize demining efforts after de-occupation is completed, and to transparently report to partners about the weapons’ efficiency. 

"It is important to note that the Russian Federation has been indiscriminately using cluster munitions from day 1 of the unprovoked large-scale aggression. In February-March 2022 Kharkiv, Ukraine’s second-largest city with over a million population, was relentlessly bombarded by Russian cluster munitions," Reznikov wrote. "Our position is simple – we need to liberate our temporarily occupied territories and save the lives of our people. For this we need to inflict losses on the enemy – war criminals, rapists and looters – who are occupying our territories. The more losses we inflict on them the more lives of Ukrainian people we will be able to save." 

Spain's defense minister answers questions from reporters

Defense Minister Margarita Robles talks to the media on July 5, 2023, in Cartagena, Spain. She condemned the U.S. decision on cluster munitions. (Martin C./Europa Press via Getty Images)

In regard to the upcoming summit in Lithuania, which Biden will attend, the U.S. and Germany insist that the focus should be on supplying weapons and ammunition to Ukraine, rather than taking the more provocative step of extending a formal invitation to join NATO. 

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Countries on NATO's eastern flank – Estonia, Latvia, Lithuania and Poland – want firmer assurances on future membership.

NATO could decide to elevate its relationship with Ukraine, creating what would be known as the NATO-Ukraine Council and giving Kyiv a seat at the table for consultations.

The Associated Press contributed to this report.