During an emergency meeting of the United Nations General Assembly last Wednesday, called in response to Russia’s invasion of Ukraine, the U.S. ambassador made an allegation about the weapons being used in the war.

“We have seen videos of Russian forces moving exceptionally lethal weaponry into Ukraine, which has no place on the battlefield,” Ambassador Linda Thomas-Greenfield said.

“That includes cluster munitions and vacuum bombs — which are banned under the Geneva Convention,” she added.

The same day, on CNN, David H. Petraeus, a retired general and former commander of U.S. Central Command, denounced Russia’s use of “missiles, rockets, artillery, cluster bombs, even these horrific thermobaric weapons that suck the oxygen out of an area and out of lungs.”

Ukraine’s ambassador to the United States, Oksana Markarova, made a similar comment the night before. “They used the vacuum bomb today,” she said, according to Reuters. “The devastation that Russia is trying to inflict on Ukraine is large.”

Russia’s invasion of Ukraine, with its constant stream of videos and photos showing devastating attacks, has reinvigorated discussions about whether certain weapons are too cruel or too indiscriminate for use in warfare.

Pentagon officials have said they are aware of reports and allegations that Russia is using cluster munitions and thermobaric weapons — which some call vacuum bombs — in Ukraine, but have been unable to confirm them. Last week, the NATO secretary general, Jens Stoltenberg, confirmed that Russia had indeed used cluster bombs.

But in calling out Russia’s actions, some officials and commentators have repeated inaccurate descriptions of some of the weapons. For example, neither cluster munitions nor thermobaric weapons are addressed in the Geneva Conventions, a series of international agreements that govern warfare.

Cluster munitions are a class of weapon comprising rockets, bombs, missiles, mortar and artillery shells that split open midair and dispense smaller weapons or bomblets over a wide area. Humanitarian groups focused on demining have noted that 20 percent or more of the anti-personnel versions of those bomblets fail to detonate on impact, yet they can explode if later picked up or handled.

The Convention on Cluster Munitions, which took effect in 2010, bans their use because of the harm they pose to noncombatants. More than 100 nations have signed the treaty, but the United States, Ukraine and Russia have not.

The weapon that Ms. Markarova referred to as a “vacuum bomb” is obscure by comparison, rarely used by modern militaries and not banned by treaties.

The warheads in most conventional bombs and rockets are designed to kill primarily by producing large amounts of lethal metal fragments when they explode. But a “vacuum bomb” — generally called a fuel-air explosive — releases an explosive slurry into the air that spreads and mixes with oxygen before exploding.

Fuel-air weapons have been used to clear minefields and landing zones of thick vegetation. But videos taken in Ukraine show that the Russian army has used them as part of an artillery weapon, a rocket system called the TOS-1A.

The Russian army’s own characterization of the TOS-1A has only added to public confusion about its nature. Russia variably refers to it as a heavy flamethrower and as a thermobaric weapon — the latter being a term the U.S. military uses to describe the kinds of munitions that have largely replaced fuel-air explosives. The new weapons have solid explosive mixtures that can produce similarly large and longer duration blast waves.

The Pentagon has used thermobaric explosives in airdropped bombs to destroy cave complexes in Afghanistan as well as in specialized hand grenades and shoulder-fired rockets meant to destroy buildings.

The term “vacuum bomb,” which refers to the weapon’s use of atmospheric oxygen and the large pressure wave it produces, is sometimes misconstrued to have an even darker meaning. Public misunderstanding about these weapons harks back to a 1975 incident in which South Vietnamese pilots were accused of using fuel-air explosive weapons during one of the final battles of the Vietnam War at Xuan Loc. That may have been the genesis of the myth that persists today — that the explosions kill people by sucking the air out of their lungs.

When asked about reports that an American-made bomb may have suffocated hundreds of victims at Xuan Loc, a Pentagon spokesman, Maj. Gen. Winant Sidle, speculated that a specific fuel-air explosive weapon might be able to consume all of the air within 20 yards of where it detonated. But if that had occurred as the general offered, atmospheric air would have quickly filled the resulting vacuum.

It is possible, however, that survivors and rescuers encountered a horrifying scene at Xuan Loc: corpses with no visible external injuries. Because fuel-air explosives produce a massive blast but relatively little attendant shrapnel, some of the dead probably would have suffered solely internal injuries.

The lungs of those victims would not have been devoid of air, but rather filled with blood after air sacs known as alveoli ruptured. The pressure could also have crushed their internal organs, which would not necessarily have left behind an obviously mangled corpse.

During the Vietnam War, activists seized on certain weapons like napalm, fuel-air explosives and cluster weapons as a way to protest the conflict in general, though their concerns did not result in any long-term changes by the Pentagon. U.S. troops used all three of those weapons in Desert Storm in 1991 and again in Afghanistan in 2001.

In 2008, Defense Secretary Robert M. Gates pledged that the Pentagon would eliminate its use of older failure-prone cluster weapons by Jan. 1, 2019. In the intervening decade, use of these weapons required approval from one of the four-star generals and admirals called combatant commanders.

In 2009, one of those commanders, General Petraeus, oversaw the last known use of cluster weapons by American forces in combat.

In late December of that year, two ships from the U.S.S. Nimitz Strike Group operating in the Northern Arabian Sea launched Tomahawk cruise missiles loaded with clusters of bomblets at a suspected Al Qaeda camp in Yemen.

Navy documents released to The New York Times show that two of the Nimitz’s escort ships engaged targets in Yemen — destroyer U.S.S. Pinckney, which launched seven Tomahawks on Dec. 24, and the cruiser U.S.S. Chosin, which launched another seven Tomahawks on Dec. 29.

In November 2017, just over a year before the U.S. military’s self-imposed ban on cluster munitions was to go into effect, Pentagon leadership reversed Mr. Gates’s policy, citing possible war with North Korea as a reason to keep the weapons available for use.

The official transcript of Ms. Thomas-Greenfield’s remarks to the U.N. General Assembly last week now contains a footnote, following questions from The Times, to reflect that the United States, Ukraine and Russia are not parties to the treaty on cluster munitions. The specific use of cluster weapons and so-called vacuum bombs “directed against civilians” is banned under the Geneva Conventions, the footnote clarifies. The language in her speech about exceptionally lethal weaponry having “no place on the battlefield” has been crossed out.

Last week, in response to a question from The Times, John F. Kirby, a Pentagon spokesman, said that the Defense Department’s cluster munitions policy was under review.