Russia has been accused of attacking Ukraine with deadly vacuum bombs. Here is how these brutal weapons work.

• Ukraine's ambassador to the US has accused Russia of using a thermobaric weapon or vacuum bomb.
• Vacuum bombs disperse fuel that mixes with oxygen, and a second charge detonates the fuel cloud.
• Overpressure from the blast creates disruptions in air spaces in the body's lung, ears, and intestines.

Ukraine's ambassador to the United States accused Russia this week of using a thermobaric weapon — a brutal weapon of war also known as a vacuum bomb — during its invasion of her country. 

The ambassador, Oksana Markarova, told reporters on Monday that Russia used a vacuum bomb the day before and said that "the devastation that Russia is trying to inflict on Ukraine is large."

A senior US defense official said on Tuesday that while Russia has launchers for this type of capability in Ukraine, it was unclear if such weapons had been used.

White House Press Secretary Jen Psaki said the US did not have confirmation that Russia had used a thermobaric weapon but said  that "if that were true, it would potentially be a war crime."

Amnesty International, a non-governmental organization dedicated to human rights, said the use of indiscriminate explosive weapons is against international humanitarian law and the use of these weapons shows "blatant disregard for civilian lives."  

Footage from inside Russia appeared to show the TOS-1 heavy flamethrower, which can shoot thermobaric rockets, mounted on a T-72 tank chassis being deployed near Ukraine on Sunday.

How does a 'vacuum bomb' work? 

Thermobaric weapons — vacuum bombs or fuel-air explosives — are a kind of two-stage munition with a container of fuel and two different explosive charges. 

The first munition is dropped or fired and the explosive charge bursts open a container, dispersing fuel in a cloud that mixes with the oxygen in the atmosphere. The second charge detonates the cloud, creating a blast wave and sucking out the surrounding oxygen like a vacuum, according to a past study by Human Rights Watch. 

The intense blast wave lasts longer than typical explosives and is capable of obliterating human bodies. The weapon is most effective in enclosed spaces like bunkers, foxholes, and reinforced buildings and on anyone wearing body armor, Human Rights Watch said.

A 1993 CIA study reported by the NGO said thermobaric weapons are particularly dangerous because they are "prone to indiscriminate use," adding that "those near the ignition point are obliterated."

Russia has previously been accused of using thermobaric weapons in Chechnya, Afghanistan and during the 1969 border dispute with China.

These weapons can still be deadly even if operated incorrectly, according to the CIA's study. If the second detonation fails, victims will inhale burning fuel. 

"If the fuel deflagrates but does not detonate, victims will be severely burned and will probably also inhale the burning fuel," the study said. "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." 

What are the medical effects of 'vacuum bombs'?

The primary injuries caused by thermobaric explosions come from the heat and pressure created by the explosion. These weapons can create pressures as high as thousands of pounds per square inch, according to an article in the Journal of Military and Veterans' Health. Normal atmospheric pressure, for comparison, is 14.7 pounds per square inch.

The significant overpressure caused by the explosion causes disruptions in air spaces of the body, including in the lungs, ears, and intestines. Fluid filling the lungs when air sacs in the organ rupture from overpressure are the most common fatal blast injury, per the Centers for Disease Control and Prevention. 

Burn injuries sustained during thermobaric explosions are more severe than conventional high-explosive blasts because of the longer duration and subsequent "after-burning" of the fuel-heated air, according to a study conducted in 2009 by English-Australian surgeon John E. Greenwood.

The study stated that secondary injuries sustained from thermobaric explosions are caused by flying glass and debris from nearby structures that interacted with the explosion and asphyxiation from the toxic gas and smoke.

Though there have been accusations of their use, it remains unclear if Russia has used these weapons in its ongoing invasion of Ukraine, but US officials have said that Russia could turn to more aggressive tactics as its invasion suffers setbacks. The use of these weapons could have a severely destructive impact if used indiscriminately on Ukrainian population centers.