Ukraine conflict: What is a vacuum or thermobaric bomb?

These are controversial because they are much more devastating than conventional explosives of similar size, and have a terrible impact on anyone caught in their blast radius.

How does a vacuum bomb work?

A vacuum bomb, also called an aerosol bomb or fuel air explosive, consists of a fuel container with two separate explosive charges.

This can be launched as a rocket or dropped as a bomb from aircraft. When it hits its target, the first explosive charge opens the container and widely scatters fuel mixture as a cloud.

This cloud can penetrate any building openings or defences that are not totally sealed. A second charge then detonates the cloud, resulting in a huge fireball, a massive blast wave and a vacuum which sucks up all surrounding oxygen. The weapon can destroy reinforced buildings, equipment and kill or injure people.
 

They are used for a variety of purposes and come in a range of sizes - including weapons for use by individual soldiers such as grenades and hand-held rocket launchers.

Huge air-launched versions have also been designed, specifically to kill defenders in caves and tunnel complexes - the effects of this weapon are at their most severe in enclosed spaces.

In 2007, Russia tested its biggest thermobaric weapon, the so-called "Father of all bombs". It created an explosion equivalent to a 44-tonne conventional bomb - making it the biggest non-nuclear explosive device in the world.

Given their devastating impact, and their usefulness against defenders who are dug in to buildings or bunkers, vacuum bombs have mainly been used in urban environments.

This is significant given developments on the ground in Ukraine, where Russian forces are trying to take control of the capital, Kyiv, and other main cities in the east of the country.

Are they being used in Ukraine?

Oksana Markarova, Ukraine's ambassador to the United States, has accused Russia of using a vacuum bomb during its invasion.

However, there has been no official confirmation of this claim.

There have also been reported sightings of thermobaric rocket launchers in Ukraine over the past few days.

What are the rules of war for vacuum bombs?

There are no international laws specifically banning their use, but if a country uses them to target civilian populations in built-up areas, schools or hospitals, then it could be convicted of a war crime under the Hague Conventions of 1899 and 1907.

International Criminal Court prosecutor Karim Khan has said his court will investigate possible war crimes in Ukraine.

Where have they been used before?

Thermobaric munitions can be traced back to World War Two, when they were initially used by the German army. They were not widely developed until the 1960s, when the US used them in Vietnam.
 

The US also used them in 2001 to try to destroy al-Qaeda forces hiding in the caves of the Tora Bora mountains of Afghanistan.

Russia used them in its war in Chechnya in 1999 and was condemned by Human Rights Watch for doing so.

Russian-made thermobaric weapons were reportedly used in the Syrian civil war by the regime of Bashar al-Assad,