A frame grab from video made available on the website of the Russian Defense Ministry on Oct. 5, 2015, shows bombs dropped by a Russian warplane. (Russian Defense Ministry Press Service via European Pressphoto Agency)

The Russian and Syrian militaries have violated international law by dropping incendiary munitions on areas populated by civilians, including recent strikes around the Syrian cities of Idlib and Aleppo, a new report by a leading human rights group said Tuesday.

The Human Rights Watch report accuses the joint Syrian-Russian air campaign of using incendiary munitions at least 18 times in the past nine weeks, most recently during an attack on Aug. 7.

“The Syrian government and Russia should immediately stop attacking civilian areas with incendiary weapons,” Steve Goose, arms director at Human Rights Watch, said in the report. “These weapons inflict horrible injuries and excruciating pain, so all countries should condemn their use in civilian areas.”

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Human Rights Watch has documented four different types of incendiary munitions used in Syria, the majority of which have been dropped by aircraft. For its most recent report, the group compiled evidence of the incendiary attacks from photographs, videos and witness statements from residents and first responders.

Incendiary weapons can be launched from both aircraft and ground-based weapons, such as artillery and mortars. Used by the United States as defoliants to torch jungle canopies, brush and sometimes villages during the Vietnam War, incendiary munitions such as napalm, phosphorous and thermite have been used intermittently, and often problematically, in conflict zones since then. Unlike traditional explosives that detonate and cause a concussive blast followed by heat and shrapnel, incendiary weapons are designed to explode and burn, creating fires that can be hard to extinguish and immolating victims caught in the weapons’ kill zone.

The presence of incendiary weapons in the Syrian conflict is not new. The weapons have been dropped sporadically — primarily by the Syrian Arab Air Force — since 2012. However, the Human Rights Watch report comes after a Russian television station in Syria filmed a pair of RBK-500 ZAB-2.5SM incendiary cluster munitions mounted to a Russian Su-34 multi-role fighter stationed at Russia’s main air base there. Earlier this month, photos posted to social media following a supposed incendiary attack showed pictures of incendiary bomb fragments with similar markings to the RBK-500s filmed on the Russian aircraft in June. In an earlier interview, Mary Wareham, the arms advocacy director at Human Rights Watch, said that the majority of the incendiary weapons used in Syria appear to be thermite-based, but they have been frequently confused with other types of incendiary munitions such as phosphorous and napalm.

Last year, the Russian Foreign Ministry acknowledged the “significant humanitarian damage” caused by incendiary weapons in Syria, which the ministry attributed to their “improper use.” Russia is party to a United Nations protocol that bans the use of airdropped incendiary munitions on areas with high concentrations of civilians. Syria, however, is not.

Russia has been bombing targets in Syria since September 2015, when it began its air campaign there to help prop up Syrian President Bashar al-Assad’s forces, saying that it is fighting terrorism. Since then the Russian air force has been repeatedly accused of targeting civilian areas, including hospitals — accusations the Russian Ministry of Defense has repeatedly denied.

Around the same time the RBK-500s were spotted in June, reports of incendiary weapons attacks on Aleppo increased. Multiple clips on YouTube show what looks like inverse fireworks blossoming downward over the city. The site of heavy fighting for months, opposition groups around Aleppo — including those backed by the United States and others deemed as hardline jihadist groups — recently managed to break through the defensive lines of Syrian government forces that had almost completely encircled the city.

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According to the Human Rights Watch report, between June 5 and Aug. 10 incendiary-based strikes wounded 12 civilians and five first responders. First responders, residents and other human rights groups reported at least 40 other incidents involving the weapons, but since no photos and videos were available Human Rights Watch could not verify the attacks. The number of casualties caused in recent weeks by incendiary weapons pales in comparison to the amount of death wrought by widely proliferated small-arms, barrel bombs, and more traditional munitions used throughout the conflict.

According to the Syria Centre for Policy Research and the United Nations, more than 400,000 have been killed since the start of the Syrian conflict in 2011.