What are cluster and vacuum weapons, and how has Russia used them in the past?

Russian forces, according to Ukraine and rights groups, also used cluster munitions in strikes on Kharkiv, the country’s second-largest city and the center of intense fighting between Ukrainian and Russian forces in recent days.

Videos shared on social media show Russian launchers for thermobaric weapons, often called “vacuum” weapons, rolling down Ukrainian streets. A CNN team reported seeing a Russian thermobaric rocket launcher south of Belgorod, Russia, near the Ukrainian border, on Saturday. And Oksana Markarova, Ukraine’s ambassador to the United States, accused Russia of using “vacuum bombs” in its invasion.

“We have seen videos of Russian forces moving exceptionally lethal weaponry into Ukraine. That includes cluster munitions and vacuum bombs — the use of which directed against civilians is banned under the Geneva Conventions,” U.S. Ambassador to the United Nations, Linda Thomas-Greenfield, said in remarks at the U.N. Wednesday.

It remains unclear whether Russia’s use of the weapons so far would constitute war crimes, since that would depend on a legal question over the extent to which Russian forces minimized risk to civilians.

A senior U.S. defense official, speaking on the condition of anonymity under ground rules set by the Biden administration, said Tuesday that Russia has launcher systems in Ukraine “that could be used for a thermobaric weapon” but that U.S. officials could not confirm the presence or use of such weapons themselves in Ukraine.

The Kremlin denied that the Russian military used cluster or vacuum munitions during the invasion.

Here’s what to know about the weapons, their legality and the threat they pose to civilians.

What to know