WASHINGTON, March 7 – As Russia’s assault on Ukraine continues, the United States has seen "very credible reports of deliberate attacks on civilians, which would constitute a war crime," Secretary of State Antony Blinken said on Sunday.

Blinken told CNN's Jake Tapper on "State of the Union" that there are also "very credible reports about the use of certain weapons." The secretary appeared to be referring to Russia’s suspected use of cluster munitions in areas of Ukraine, killing and wounding civilians.

Ukraine’s ambassador to the United States, Oksana Markarova, accused Russia on February 28 of using vacuum bombs, controversial thermobaric weapons, in its invasion. A CNN team reported seeing a Russian thermobaric rocket launcher — which is capable of launching vacuum bombs — south of Belgorod, Russia, near the Ukrainian border, on Feb. 26, but so far there is no evidence that the actual thermobaric weapons have been used by Russia. The Kremlin denied the use of cluster munitions or vacuum bombs.

These are the types of weapons that could be part of the International Criminal Court’s investigation that was launched last week into allegations of war crimes by Russia.

To understand what cluster munitions and vacuum bombs are, the kind of destruction they cause and why they’re controversial, Yahoo News spoke to David, a principal researcher at the RAND Corporation, an American global policy think tank. The interview has been shortened and edited for clarity.

"Vacuum bomb" is not a particularly good description. It's a thermobaric weapon. And nobody ever hears the word "thermobaric" very much, so it's confusing. I think the difference between thermobaric systems that are either single-warhead or cluster munitions is that the way a thermobaric warhead works is, there is a two-stage system, Johnson said.

“When it first hits, it flows the aerosol and essentially completely blankets the range of the system in this vapor. The second stage ignites it. And so it may feel like a person is standing in a room and [it] also fills the room full of natural gas or gasoline that's been vaporized, and throwing a match into it,” Johnson said.

Russia is believed to have 28 220-mm rockets, and the only purpose of those is to shoot thermobaric rounds. One launcher with these 28 things can launch in 15 to 30 seconds, all the rockets in it. And it will essentially blanket an area that's 200 by 400 meters. (yn/ez)