Ukraine's ambassador to the United States reportedly said Russia used a vacuum bomb during its invasion of the country and rights groups have accused Russia of using cluster bombs during the attacks, but what are these bombs and how are they different from other explosives?
Here's a breakdown of what to know about vacuum bombs, thermobaric bombs and cluster bombs amid Russia's invasion of Ukraine.
What are vacuum bombs or thermobaric bombs?
A thermobaric bomb, also known as a vacuum bomb, uses oxygen from its surrounding area to generate a longer-lasting blast.
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According to Reuters, the bombs are capable of "vaporizing human bodies."
What about cluster bombs?
Cluster bombs are weapons that open in the air, releasing submunitions, or “bomblets,” that are dispersed over a large area, intended to wreak destruction on multiple targets at once. Cluster bombs can be delivered by planes, artillery and missiles, according to the International Committee of the Red Cross.
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Beyond the initial harm caused by the munitions upon impact, bomblets have a high rate of failure to explode, up to 40% in some recent conflicts, according to the ICRC.
That leaves swaths of land dotted with bomblets that could explode. Return to normal life in those areas becomes hazardous, particularly in heavily populated areas. Some formerly war-torn countries spend years trying to clear unexploded cluster bomblets.
What has and has not been confirmed?
Oksana Markarova, Ukraine's ambassador to the United States, told reporters after meeting with members of the U.S. Congress that Russia had used a thermobaric weapon during its invasion.
"They used the vacuum bomb today, which is actually prohibited by the Geneva convention," Markarova said. "The devastation that Russia is trying to inflict on Ukraine is large."
She did not provide specifics on where such a weapon was used nor did she provide evidence to support her claim.
Dmitry Peskov, a Russian spokesperson, denied the allegations Tuesday, saying it's "definitely a fake."
White House Press Secretary Jen Psaki said during a briefing that she had seen reports of vacuum bombs being used but could not confirm.
"We have seen the reports," she said. "If that were true it would potentially be a war crime."
CNN reported its team had seen a thermobaric "launcher" near the Ukrainian border over the weekend but noted there was no evidence one had been used.
Rights groups and observers also said Russia is using cluster bombs in its invasion of Ukraine, a charge Moscow denies. If confirmed, deployment of the weapon, especially in crowded civilian areas, would usher in new humanitarian concerns in the conflict, Europe’s largest ground war in generations.
Human Rights Watch’s associate arms director Mark Hiznay told The Associated Press Russian forces have “most definitely” used cluster bombs in Ukraine.
He pointed to at least two instances: a missile attack that hit outside a hospital in the town of Vuhledar on the first day of the invasion last week and another Monday on Ukraine's second largest city, Kharkiv, with a population of 1.4 million.
Hiznay retweeted photos of what he said was unexploded 9N235 cluster submunitions scattered around Kharkiv. Human Rights Watch said four people were killed in the Vuhledar strike.
Rights groups have said three people were killed after Russian cluster bombs hit near a pre-school in the northeastern city of Okhtyrka. The open-source intelligence group Bellingcat says that its researchers found cluster munitions in that strike as well as multiple cluster attacks in Kharkiv.
Amnesty International said Russian forces have a “shameful record of using cluster munitions in populated areas.”
Justin Bronk, a research fellow at Royal United Services Institute, a London defense think tank, said images of munitions parts recovered from residential areas of Kharkiv are “concrete evidence” that Russia is using cluster bombs.
“Their use suggests the Russians are trying to break morale, inflict terror on the on the civilian population and the defenders in order to try and force a negotiation or just retreat,” he said.
Russia denies using cluster munitions in Ukraine.
Is the use of cluster bombs legal?
Use of cluster bombs itself does not violate international law, but using them against civilians can be a violation. As in any strike, determining a war crime requires looking at whether the target was legitimate and if precautions were taken to avoid civilian casualties.
“The part of international law where this starts playing (a role), though is indiscriminate attacks targeting civilians,” Human Rights Watch’s associate arms director Mark Hiznay told The Associated Press. “So that’s not necessarily related to the weapons, but the way the weapons are used.”
A convention banning the use of cluster bombs has been joined by more than 120 countries who agreed not to use, produce, transfer or stockpile the weapons and to clear them after they’ve been used.
Russia and Ukraine have not joined that convention. Neither has the United States.
Proponents of banning cluster bombs say they kill indiscriminately and endanger civilians long after their use. From Syria and Yemen to the Balkans, Afghanistan and Southeast Asia, unexploded ordnance from cluster bombs continues to kill and maim people years or even decades after the munitions were fired.
Though many countries have joined a global convention limiting their use, cluster munitions are still used in conflict zones around the world.
What does the development mean for Ukraine?
U.S. senators noted Ukraine's need for added weapons as they emerged from a Monday evening meeting with Markarova at the Capitol. Congress is preparing supplemental funding to help Ukraine during the crisis and the White House is seeking at least $6.4 billion in military and humanitarian aid.
“They need more arms,” said Sen. Mark Warner, D-Va., the chairman of the Intelligence Committee.
“It’s David versus Goliath,” said Sen. Jim Risch of Idaho, the top Republican on the Foreign Relations Committee. “I think that any human being reading the reports coming out of there realize that this is dire.”
Senators in the U.S. are working to provide ammunition such as anti-tank and anti-aircraft systems to Ukraine -- what Risch called an “all of the above” effort.
Ukrainian President Volodymyr Zelenskyy says Russian troops have intensified shelling of Ukraine, calling it an effort to force his government into making concessions during talks held Monday.
In a video address late Monday, Zelenskyy said that “the talks were taking place against the backdrop of bombing and shelling of our territory, our cities. Synchronizing of the shelling with the negotiating process was obvious. I believe Russia is trying to put pressure (on Ukraine) with this simple method.”
The president gave no details about the hours-long talks themselves. But he says Ukraine is not prepared to make concessions “when one side is hitting each other with rocket artillery.”
Zelenskyy says that Kyiv, the capital, remains “a key goal” for the Russians and that Russian forces have also shelled the city of Kharkiv with rocket artillery.