Washington, DC – The United States has authorised the transfer of cluster munitions to Ukraine against the objections of rights advocates who have been calling for a ban on the weapons, which they say endanger civilians.

The administration of President Joe Biden confirmed the move on Friday, arguing that US-made cluster bombs are safer than the ones Russia is already using in the conflict. The transfer comes as Ukraine pushes on with a counteroffensive against Russian troops in the east of the country.

“We recognise that cluster munitions create a risk of civilian harm from unexploded ordnance,” US national security adviser Jake Sullivan told reporters.

“This is why we’ve 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.”

Sullivan added that cluster munitions would serve as a “bridge” to supplement conventional artillery as the US ramps up production of regular bombs and shells for Ukraine.

Biden later told CNN that it was a “very difficult decision” on his part, adding that the “Ukrainians are running out of ammunition”.

The weapons are part of a tranche of US military assistance to Ukraine that also includes armoured vehicles and anti-armour weapons, the Pentagon announced.

Rights advocates slammed the Biden administration’s decision, highlighting their threat to Ukrainian civilians.

Sarah Yager, the Washington director at Human Rights Watch, called the US move “devastating”.

“They are absolutely awful for civilians.” Yager told Al Jazeera in a television interview. “I think when legislators and policymakers here in the United States see the photos coming back of children with missing limbs, parents injured, killed by our own American cluster munitions, there’s going to be a real awakening to the humanitarian disaster that this is.”

UN opposition

Each cluster bomb can contain hundreds of smaller explosives that spread across a targeted area, but not all of these bomblets detonate on impact. The unexploded bombs, known as duds, can remain embedded in the ground for years, posing a serious danger to civilians, most notably children.

While cluster munitions are not banned internationally, more than 120 countries – including most NATO members – have signed on to a convention prohibiting their use. The US, Ukraine and Russia are not party to that agreement.

On Friday, Farhan Haq, a spokesperson for United Nations Secretary-General Antonio Guterres, reiterated the UN chief’s support for the Convention on Cluster Munitions.

“He wants countries to abide by the terms of that convention, and so as a result, of course, he does not want there to be continued use of cluster munitions on the battlefield,” Haq said.

Germany, a NATO member and top Ukraine ally, has also voiced opposition to sending cluster munitions to Ukraine.

But NATO Secretary-General Jens Stoltenberg suggested that the alliance does not take a position on the issue, leaving it to the individual states to make their own policies.

“Cluster munition is already in use in the war on both sides. The difference is that Russia uses the cluster munitions in a war of aggression to occupy, to control, to invade Ukraine while Ukraine is using it to defend itself against aggression,” Stoltenberg told Al Jazeera’s diplomatic editor James Bays in an interview.

But Yager dismissed the argument that Russia’s use of the weapons justifies further deployment of cluster munitions by Ukraine. “The fact that Russia is using them is just another reason why they should not be used,” she said.

With the US providing military support, Ukraine has been making only modest gains in a much-anticipated counteroffensive launched last month.

Last year, US envoy to the UN Linda Thomas-Greenfield slammed Russia for using “exceptionally lethal” weapons in Ukraine, including cluster munitions.

On Friday, Sullivan said Washington has “written assurances” from Kyiv that it would use the cluster munitions in a careful way to minimise civilian harm.

How cluster munitions work

Patrick Fruchet, a landmine clearance expert, said explosive remnants of war – bombs that “fail to go bang” when launched – are a major source of risk in conflict areas.

Fruchet said the main concern with cluster munitions is their failure rate and their “twitchy” qualities, which makes the unexploded devices vulnerable to detonation when handled.

“You see a lot of children coming upon novel-looking devices and being attracted to them because they’re unusual, … and there’s a tendency to pick them up,” he said.

The Pentagon said on Thursday that the cluster bombs it is considering providing to Ukraine have a dud rate lower than 2.35 percent.

But Fruchet said the dud estimate on the explosives are unreliable, citing his experience with the UN Mine Action Service in Afghanistan, where he dealt with cluster bombs with a supposed failure rate of 5 percent.

“The teams on the ground, we saw failure rates of up to 40 percent based on going out and clearing the space – knowing how many cluster munitions would have been in one clamshell and then basically counting how many we had to clear,” he said.

BREAKING: Amnesty International opposes the transfer of cluster munitions to the armed forces of Ukraine

Daniel Balson, @AmnestyUSA’s Advocacy Director for Europe and Central Asia said:https://t.co/WEaSFb9IJA

— Amnesty International USA (@amnestyusa) July 6, 2023

Unlike landmines, cluster bombs are not designed to be triggered by the proximity of people or vehicles; they are meant to explode when dropped. But once left unexploded, the bomblets “function in practice very similarly to landmines” – they go off if they are disturbed, Fruchet told Al Jazeera.

The duds can still detonate decades after they are dropped. “There’s no reason to believe that they ever really become inert, that they ever become harmless,” Fruchet said. “These things are made to an industrial standard. They’re often stored for a long time.”

For example in south Lebanon, cluster munitions fired by Israel during the 2006 war continue to endanger civilians today.

On Friday, Amnesty International slammed the US decision and called on Washington to reconsider its policy.

“The USA’s plan to transfer cluster munitions to Ukraine is a retrograde step, which undermines the considerable advances made by the international community in its attempts to protect civilians from such dangers both during and after armed conflicts,” the group said in a statement.