An advanced Russian cluster munition was used in an airstrike southwest of Aleppo on October 4, Human Rights Watch reported on Saturday.

The New York-based watchdog expressed concerns that Russia either used cluster munitions in Syria or provided the Syrian army with deadly SPBE sensor fused submunitions in a reported attack October 4 attack in the village of Kafr Halad.

“It’s disturbing that yet another type of cluster bomb is being used in Syria given the harm they cause to civilians for years to come. Neither Russia nor Syria should use cluster munitions, and both should join the international ban without delay”, Human Rights Watch’s deputy Middle East director, Nadim Houry said.

The evidence came from a careful examination of photographs and videos from various sources. The SPBE fused submunitions in Kafr Halab most likely deployed from an RBK-500 SPBE cluster bomb, which is the only type of airdropped bomb capable of delivering this weapon.

A cluster bomb can house multiple explosive submunitions, such as landmines, bomblets and grenades. They can be fired from the ground or dropped from an aircraft, and scattered over a wide area.

Like traditional landmines, cluster bombs can pose a threat for years after the end of an armed conflict, because their submunitions often fail to explode on deployment.

The report said that the attack marked the first time the SPBE cluster bomb was used in Syria, since the beginning of the civil war. Human Rights Watch has documented the use of cluster munitions in the war in Syria by the Syrian government since 2012 and by Islamic State since the second half of 2014. No other group reported to have used cluster weapons in Syria.

The report couldn’t determine whether Russia or Syrian forces used the weapon.

An international treaty, 2008 Convention on Cluster Munitions, prohibits the use, production, transfer, and stockpiling of the deadly weapon. Neither Russia nor Syria is among the 98 signatories of the treaty.

Russia began airstrikes in Syria on September 30, under the request of the Syria President Bashar Assad. Moscow says it is targeting Islamic State, however, it has also heavily hit rebel-held areas on the frontline against Syria’s army. This raise concerns in the West that Russia’s motivation behind its intervention is to boost Assad’s hold on power. Russia denied these allegations, saying its airstrikes are solely targeting militant Islamists.

According to the data collected by the Cluster Munition Coalition, of which Human Rights Watch is a cofounder, there were about 2,000 casualties from cluster bombs in Syria from 2012 until the end of 2014. The majority of those killed were civilians.