Rishi Sunak has ruled out supplying Ukraine with cluster bombs, saying the UK will not follow the Biden administration’s controversial move and will instead press countries to boost their aid to Kyiv “in other ways”. On Friday, Joe Biden defended what he said was a “difficult decision” to send widely banned cluster munitions to Volodymyr Zelenskiy’s government. Human rights groups criticised the White House and there was unease among some Democrats, with one calling it “unnecessary and a terrible mistake”. On Saturday, the prime minister pointed out the UK was a signatory to an international convention that discourages the use and production of cluster munitions – unlike the US, Russia and Ukraine. Sunak said: “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. “Russia’s act of barbarism is causing untold suffering to millions of people.” Sunak said he was travelling this week to the Nato summit in Vilnius, Lithuania, and would discuss with allies how to “strengthen our support for Ukraine”. It is unclear if Zelenskiy will attend in person. He has been pressing for immediate Nato membership for Ukraine, something Biden has so far ruled out. The US and other countries including Germany are fearful that admitting Ukraine into Nato now – 16 months after Vladimir Putin’s invasion – will escalate tensions with Moscow. Nato members are instead likely to offer Kyiv a number of “security guarantees” as a sign of their long-term commitment. Zelenskiy has welcomed the US provision of cluster munitions, which are part of a new $800m (£625m) security package. It comes amid western concerns that Ukraine’s summer counteroffensive is making slow progress. Commanders say their dug-in Russian opponents have superiority in artillery and heavy tanks, limiting the ability of their troops to advance. Cluster munitions are prohibited by more than 100 countries. They typically scatter numerous smaller bomblets over a wide area, sometimes as big as a football pitch, and can kill indiscriminately. Those that fail to explode threaten civilians, especially children, for decades after a conflict ends. Moscow has used cluster munitions extensively, dropping them on the Ukrainian city of Kharkiv and in other heavily populated areas. Ukraine’s defence minister, Oleksii Reznikov, described the US-supplied munitions as “liberation weapons” that would “significantly help us to de-occupy our territories while saving the lives of Ukrainian soldiers”. Reznikov said he had given Washington written and “personal” guarantees. He promised his armed forces would not use cluster weapons inside Russia, and would avoid bombing urban areas where Ukrainian civilians could be killed or injured. “These are our people,” he said in a tweet. He explained: “Cluster munitions will be used only in the fields where there is a concentration of Russian military. They will be used to break through the enemy defence lines with minimum risk for the lives of our soldiers.” Kyiv would also keep a record of where it had fired cluster shells, known in Ukraine as “cassette bombs”. Based on these records, it would seek to de-mine affected areas as a priority once Ukraine had defeated Russia, Reznikov wrote. Speaking on Friday, Biden said he had made the move because the Ukrainians were running out of ammunition. In an interview with CNN, the US president said: “This is a war relating to munitions. And they’re running out of that ammunition, and we’re low on it and so, what I finally did, I took the recommendation of the defense department to – not permanently – but to allow for this transition period, while we get more 155 weapons, these shells, for the Ukrainians.” Jake Sullivan, Biden’s national security adviser, told reporters at the White House: “We recognise that cluster munitions create a risk of civilian harm from unexploded ordnance. This is why we deferred the decision for as long as we could. But there is also 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 because Ukraine does not have enough artillery. “That is intolerable to us. Ukraine would not be using these munitions in some foreign land. This is their country they’re defending. These are their citizens they’re protecting and they are motivated to use any weapon system they have in a way that minimises risks to those citizens.” Zelenskiy described the aid package as “timely, broad and much-needed defense”, in a tweet thanking Biden for “decisive steps that bring Ukraine closer to victory over the enemy”. “The expansion of Ukraine’s defense capabilities will provide new tools for the de-occupation of our land and bringing peace closer,” he wrote.