Aircraft from either the Syrian or Russian air force dropped incendiary bombs on civilian targets in two Syrian provinces Monday, the Observatory for Human Rights reports. The attack comes fresh off considerable international attention on the Syrian Civil War following the American cruise missile attack on a Syrian military airfield on Friday. That attack was in response to the Assad regime’s use of chemical weapons on civilian targets last week.

Activists and workers on the ground reported that the weapons used were thermite bombs, a weapon that has been commonly used by Assad forces and their Russian allies throughout the war. Thermite is one of the hottest-burning substances on Earth and has many uses. Because it does not explode, it has been used by militaries for decades as one method of permanently disabling pieces of equipment without needing explosives. Thermite can also be used to clear congested areas and deny positions of cover and concealment, such as in a thickly wooded forest. The weapon has a similar effect in urban areas, where it has been used successfully in Syria.

In 2016, the media propaganda mouthpiece for the Russian government known as RT accidently showed Russian jets armed with thermite bombs. The images were quickly edited away, but their appearance in the photographs directly correlated to the use of thermite bombs on civilian targets in Aleppo shortly thereafter.

Although any weapon used intentionally on civilian targets constitutes a war crime, the use of incendiary weapons like thermite is especially heinous, as it causes incredibly grievous wounds. The fires it starts are also difficult to extinguish—another reason the Russians have made use of it as an area-denial weapon.

The United Nations Convention on Conventional Weapons expressly prohibits the use of air-dropped incendiary weapons on civilian areas. Although Russia is a signatory to that convention, Bashar al-Assad’s Syria is not, and is quite comfortable with violating numerous international arms agreements.

Although thermite bombs have been used somewhat intermittently since the start of the war, their use so quickly after a U.S. strike against the Syrian military is possibly Assad’s own messaging back to the United States and the international community.

White House Press Secretary Sean Spicer raised eyebrows Monday after saying, “If you gas a baby, if you put a barrel bomb into innocent people, I think you will see a response from this president.” The implication of this seems to be that the threshold for direct U.S. engagement against Assad forces had been lowered to include attacks on civilians.

Spicer clarified the comment by saying the “president retains the option to act in Syria against the Assad regime whenever it is in the national interest.”