What is a thermobaric bomb?

Russia has been accused of using thermobaric vacuum bombs during its invasion of Ukraine, according to Ukrainian officials.

"They used the vacuum bomb today," Ukraine's ambassador to the United States Oksana Markarova told reporters after meeting with U.S. lawmakers Monday. "...The devastation that Russia is trying to inflict on Ukraine is large."

MOSCOW, RUSSIA - 2021/08/25: Russian Army TOS-1A "Solntsepyok (Blazing Sun)" multiple rocket launcher and thermobaric weapon mounted on a T-72 tank chassis seen during the annual Army Games defense technology international exhibition. The International Army Games is an annual Russian military sports event organized by the Ministry of Defense of Russia. The event, which was first staged in August 2015, involves close to 30 countries taking part in dozens of competitions over two weeks to prove which is the most skilled. Dynamic demonstration is a part of the Army Games public display. (Photo by Leonid Faerberg/SOPA Images/LightRocket via Getty Images) (Getty)

Thermobaric explosives, also called fuel-air explosives (FAE), use oxygen around them to generate a high-temperature blast. The blast wave typically lasts longer than a conventional explosive and has the ability to vaporize people.

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"The [blast] kill mechanism against living targets is unique--and unpleasant...." reads a 1993 U.S. Defense Intelligence Agency study. "What kills is the pressure wave, and more importantly, the subsequent rarefaction [vacuum], which ruptures the lungs.... If the fuel deflagrates but does not detonate, victims will be severely burned and will probably also inhale the burning fuel. Since the most common FAE fuels, ethylene oxide and propylene oxide, are highly toxic, undetonated FAE should prove as lethal to personnel caught within the cloud as most chemical agents."

Using such explosives is a breach of the Geneva Convention, which established guidelines for humanitarian treatment of populations in war.

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Russia has a long history with the explosives. In 1999, Russian generals threatened to use vacuum bombs on rebels during the Chechnya conflict. While in 2007, Russian officials announced the country had tested the world's most powerful vacuum bomb.

"Test results of the new airborne weapon have shown that its efficiency and power is commensurate with a nuclear weapon," Alexander Rukshin, then-deputy head of Russia's armed force chief of staff, told Russian TV, Reuters reported at the time.

"You will now see it in action, the bomb which has no match in the world is being tested at a military site," the report said.

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Russia has continued pummeling certain cities in Ukraine since it first invaded the nation on Thursday. Near Kyiv, Satellite imagery shows a roughly 40-mile long Russian convoy composed of tanks and armored vehicles. Russian state media warned Tuesday of imminent attacks that will allegedly target the Security Service of Ukraine (SBU) and 72nd Main Centre for Psychological Special Operations (PSO).

Smoke rising after shelling on the outskirts of the city is pictured from central Kyiv, Ukraine February 27, 2022. REUTERS/Irakli Gedenidze (REUTERS/Irakli Gedenidze)

A view shows a burning oil depot reportedly hit by shelling near the military airbase Vasylkiv in the Kyiv region, Ukraine February 27, 2022. REUTERS/Maksim Levin (REUTERS/Maksim Levin)

Smoke rising after shelling on the outskirts of the city is pictured from Kyiv, Ukraine February 27, 2022. REUTERS/Mykhailo Markiv ( REUTERS/Mykhailo Markiv)

In Ukraine’s second-largest city, Kharkiv, shelling has intensified, including with an attack on the city’s main square.

A destroyed armored vehicle is seen in front of a school which, according to local residents, was on fire after shelling, as Russia's invasion of Ukraine continues, in Kharkiv, Ukraine, Feb. 28, 2022. REUTERS/Vitaliy Gnidyi (REUTERS/Vitaliy Gnidyi)

An armored personnel carrier burns and damaged light utility vehicles stand abandoned after fighting in Kharkiv, Ukraine, on Sunday. The city authorities said that Ukrainian forces engaged in fighting with Russian troops that entered the country's second-largest city on Sunday. (AP)

A Ukrainian servicemen opens the door of a deactivated Russian military multiple rocket launcher on the outskirts of Kharkiv, Ukraine, Friday, Feb. 25, 2022. Russian troops bore down on Ukraine's capital Friday, with gunfire and explosions resonating ever closer to the government quarter, in an invasion of a democratic country that has fueled fears of wider war in Europe and triggered worldwide efforts to make Russia stop. ( (AP Photo/Vadim Ghirda))

Ukrainian President Volodymyr Zelenskyy called the attack on Kharkiv "frank, undisguised terror," saying a Russian missile was to blame and that the incident amounted to a war crime.

Zelenskyy has called for an international tribunal to investigate Russia’s actions and for Russia to be penalized through measures such as losing its permanent membership on the United Nations Security Council and closing airspace, ports and canals to Russian vehicles.

The Ukrainian president signed an application Monday for Ukraine to join the European Union. The EU has accepted Ukraine’s application and has commenced a special admission procedure to integrate the country, Eastern European media has reported.

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"We are fighting also to be equal members of Europe. I believe that today we are showing everybody that is what we are … We have proven that, as a minimum, we are the same as you."

The Associated Press and Fox News’ Peter Aitken contributed to this report.