Since the start of the seven-month Israel-Hamas conflict, powerful US-supplied 2,000-pound bombs have been used in bombardments on Gaza's heavily populated cities.
Now, for the first time, US President Joe Biden has acknowledged that the bombs, which military experts say turn "earth into liquid", have killed civilians in Gaza.
And the US will be delaying a shipment of thousands of bombs over concerns about Israel's invasion of the southern city of Rafah.
"Civilians have been killed in Gaza as a consequence of those bombs and other ways in which they go after population centres," Mr Biden told CNN.
"I made it clear that if they go into Rafah ... we're not going to supply the weapons and artillery shells used."
Analysts say it's a "powerful symbolic move" from the Biden administration.
But, it's unlikely to have an impact on the conflict overall, or Israeli Prime Minister Benjamin Netanyahu's planned assault on Rafah.
So, what does the move tell us?
Why these bombs?
Washington has paused one shipment consisting of 1,800 MK84 2,000-pound bombs, and 1,700 500-pound bombs, according to US officials.
Four sources said the shipment, which has been delayed for at least two weeks, involved Boeing-made Joint Direct Attack Munitions, which convert dumb bombs into precision-guided ones, as well as Small Diameter Bombs.
They were part of an earlier approved shipment to Israel, not the recent $US95 billion supplemental aid package the US Congress passed in April.
The blast of an MK84 bomb is so immense "it turns earth into liquid", Marc Garlasco, a former Pentagon official and a war crimes investigator for the United Nations told the Associated Press last month.
"It pancakes entire buildings," he said.
He added an explosion in the open meant "instant death" for anyone within about 30 metres.
Lethal fragmentation can extend for up to 365m.
There have been concerns raised about the use of the bombs and their links to civilian deaths.
Amnesty International has called for an investigation into Israel's use of heavy weapons such as the MK84.
But it has taken the looming full-scale assault on Rafah, which is where more than a million Palestinian civilians have been sheltering, for the US to reconsider their supply.
Defence Secretary Lloyd Austin confirmed the weapons delay late on Wednesday, local time, telling the Senate Appropriations Subcommittee on Defense that the US is reviewing "near term security assistance in the context of unfolding events in Rafah".
"We've been very clear ... from the very beginning that Israel shouldn't launch a major attack into Rafah without accounting for and protecting the civilians that are in that battle space," he said.
Michael Shoebridge, director and founder of Strategic Analysis Australia, says the weapons being blocked only make up a fraction of the military assistance the US is providing Israel.
"So pausing a supply of 3,000 bombs and guidance kits won't affect Israel's ability to defend itself," he told the ABC.
"Nor will it stop the Israel Defense Forces from operating in Rafah."
Biden showing 'he has limits'
The US has made it clear that although this particular weapons shipment would be put on pause, its support for Israel remains strong.
"We're not walking away from Israel's security," Mr Biden said.
"We're walking away from Israel's ability to wage war in those areas."
Washington has opposed a major Israeli invasion of Rafah without civilian safeguards.
Israeli forces on Tuesday seized the main border crossing between Gaza and Egypt in Rafah, cutting off a vital route for aid into the tiny enclave.
Mr Biden said Israel's actions around Rafah had "not yet" crossed his red lines, but has repeated that Israel needs to do far more to protect the lives of civilians in Gaza.
Mr Shoebridge believes the US is serious about wanting Israel to change the way it approaches its assault on Rafah.
Mr Biden is also facing increasing domestic pressure around the safety of civilians in Gaza in the lead up the the 2024 presidential elections.
"It's a powerful symbolic move between the Biden administration and the Israeli government," Mr Shoebridge said.
"This is an adjustment from moving beyond trying to convince Netanyahu with words to showing Biden means it when he says he has limits."
Will Israel listen?
Israel denies targeting Palestinian civilians, saying its sole interest is to annihilate Hamas and that it takes all precautions to avoid unnecessary death.
More than 34,7000 Palestinians — most of them civilians — have been killed in the conflict, according to the Gaza Health Ministry.
After news broke about the weapons blocks, a senior Israeli official, speaking to Reuters on condition of anonymity, refused to confirm the reports but also appeared unfazed.
"As the prime minister [Netanyahu] has already said, if we have to fight with our fingernails, then we'll do what we have to do," the source said.
The Biden administration has previously taken smaller steps to show displeasure with Mr Netanyahu, including imposing sanctions on Israeli settlers and letting through a UN Security Council resolution that supported a ceasefire.
Raphael Cohen, director of the strategy and doctrine program at the RAND Corporation research group, told AFP news agency that despite Mr Netanyahu's rhetoric, "Israel takes American pressure quite seriously".
But avoiding a Rafah invasion "functionally means leaving at least four battalions of Hamas fighters plus its senior leadership intact and over 100 hostages in Hamas hands," he said.
"From an Israeli strategic perspective, that's probably a nonstarter and it also may fracture Netanyahu's coalition."
About 133 hostages are believed to remain in captivity in Gaza, according to Israeli tallies, following Hamas' October 7 attack, which killed about 1,200 people with about 250 others abducted.
Is this the end of heavy bombs?
There has been heated debate around the use of heavy weapons such as the MK84 bombs in Gaza over the risk of large numbers of civilian deaths.
International humanitarian law does not explicitly ban aerial bombing in densely populated areas, however civilians cannot be targets and a specific military aim must be proportionate to possible civilian casualties or damage.
The statute of the International Criminal Court, which is investigating the Israel-Gaza war, lists as a war crime intentionally launching an attack when it is known that civilian death or damage will be "clearly excessive" compared to any direct military advantage.
Mr Shoebridge says the US is unlikely to admit fault in its previous supply of 2,000-pound bombs to Israel.
The move regarding Rafah will be seen as "forward-looking rather than backward-looking," he said.
"I think the US position remains that proportionality is the test in war, and proportionality doesn't rule out damage to civilian infrastructure and damage or death in the civilian population," he added.
"So I don't think the US position on that will have changed."
Independent Senator Bernie Sanders, a Biden ally, said in a statement that the pause on big bombs must be a "first step" in reconsidering military aid to Israel.
Mr Austin, meanwhile, told the Senate that "it's about having the right kinds of weapons for the task at hand".
"A small diametre bomb, which is a precision weapon, that's very useful in a dense, built-up environment," he said.
"But maybe not so much a 2,000-pound bomb that could create a lot of collateral damage."
He added the US wants to see Israel do "more precise" operations.
ABC/Wires
