US President Joe Biden has defended his “very difficult decision” to give Ukraine cluster bombs, which have a record of killing civilians.
The president said it had taken him “a while to be convinced to do it”, but he had acted because “the Ukrainians are running out of ammunition”.
Ukraine’s leader hailed the “timely” move, but human rights groups and some Democrats criticised the decision.
A Moscow envoy slammed Washington’s “cynicism”.
Mr Biden told CNN in an interview on Friday that he had spoken to allies about the decision, which comes ahead of a Nato summit in Lithuania next week.
Cluster bombs are banned by more than 120 countries, but have been used by both Russia and Ukraine during the war.
“This is why we’ve deferred the decision for as long as we could.”
Mr Sullivan said Ukraine was running out of artillery and needed “a bridge of supplies” while the US ramps up domestic production.
“We will not leave Ukraine defenceless at any point in this conflict period,” he said.
The munitions have caused controversy over their failure – or dud – rate, meaning unexploded small bombs can linger on the ground for years and indiscriminately detonate later on.
Mr Sullivan told reporters the American cluster bombs being sent to Ukraine were far safer than those he said were already being used by Russia in the conflict.
Mr Biden’s move will bypass US law prohibiting the production, use or transfer of cluster munitions with a failure rate of more than 1%.
Early on in the war, when the White House was asked about allegations that Russia was using cluster and vacuum bombs, the then-press secretary said it would be a potential “war crime” if true.
Marta Hurtado, speaking for the UN human rights office, said on Friday: “The use of such munitions should stop immediately and not be used in any place.”
Russia’s ambassador to the US criticised Mr Biden’s decision.
“The cruelty and cynicism with which Washington has approached the issue of transferring lethal weapons to Kyiv is striking,” Anatoly Antonov was quoted as saying by Tass news agency.
Russian President Vladimir Putin has previously accused the US and its allies of fighting an expanding proxy war in Ukraine.
But Ukrainian President Volodymyr Zelensky thanked the US president for “a timely, broad and much-needed” military aid package worth $800m (£626m).
He tweeted that it would “bring Ukraine closer to victory over the enemy, and democracy to victory over dictatorship”.
Ukraine’s counter-offensive, which began last month, is grinding on in the eastern Donetsk and south-eastern Zaporizhzhia regions.
Last week, Ukraine’s military commander-in-chief Valery Zaluzhny said the campaign had been hampered by a lack of adequate firepower, and expressed frustration with the slow deliveries of weapons promised by the West.
Pentagon spokesman Colin Kahl said giving Ukraine cluster munitions would let “the Russians know that the Ukrainians are going to stay in the game” when it comes to the war.
Human rights groups criticised the decision, with Amnesty International saying cluster munitions pose “a grave threat to civilian lives, even long after the conflict has ended”.
The US Cluster Munition Coalition, which is part of an international civil society campaign working to eradicate the weapons, said they would cause “greater suffering, today and for decades to come”.
Reaction from US lawmakers on Capitol Hill to the decision to send cluster munitions to Ukraine has been mixed, with some Democrats labelling the decision “alarming” and “a terrible mistake”.
Adam Smith, a Washington Democrat on the House Armed Services Committee, had a different view – telling the BBC the White House had made the “right call”.
“Making sure that the Ukrainians can retake their territory is one of the most important things we can do to help reduce civilian casualties,” he said.
In a joint statement, Republican leaders on the House and Senate foreign affairs committees said the Democratic president’s move would allow Ukraine’s forces “to target and eliminate Russian forces more efficiently”.
Nato Secretary General Jens Stoltenberg said the military alliance took no position on cluster munitions.
More than 120 countries have committed to the Convention on Cluster Munitions, agreeing not to use, produce, transfer or store such devices. The US, Ukraine and Russia are not party to the agreement.
Germany, which has signed the ban treaty, said it would not provide such bombs to Ukraine, but expressed understanding for the American position.
“We’re certain that our US friends didn’t take the decision about supplying such ammunition lightly,” German government spokesman Steffen Hebestreit told reporters in Berlin.
“We need to remember once again that Russia has already used cluster ammunition at a large scale in its illegal war of aggression against Ukraine.”
Human Rights Watch said both sides had used the weapons in the war, causing “numerous deaths and serious injuries to civilians”.