Thermobaric weapons are also called vacuum bombs as they suck the oxygen out of the environment and the explosion lasts longer than "ordinary" explosions.
They are often described as very relentless weapons.
Sergei Bratchuk, Ukrainian spokesman for the Odessa region, says a TOS-1A weapons system - previously discovered in Russian military convoys during the failed attack on Kyiv - has been fired at Russian troops near Izyum in eastern Ukraine, according to The Times.
The British expert Dan Smith, head of Stockholm's international peace research institute Sipri, says that it is not possible to determine whether the information is true.
But he considers it likely that Ukraine conquered the Russian weapons and used them.
- It's credible.
It is not hard to believe that when Russian forces withdrew, Ukrainian forces took one or more in good enough condition to be used, says Dan Smith to SVT.
He continues:
- TOS-1A specializes in firing a thermobaric weapon and I have not seen references to it being able to fire other types of projectiles.
Thermobaric weapons have previously been used by the US military against Taliban and al-Qaeda bases in caves in Afghanistan.