Human Rights Watch on Sunday accused Russia of being behind the use of new advanced cluster munitions in Syria, by dropping them from its warplanes or supplying them to the Damascus regime.
The New York-based group said that photographs it obtained showed that the cluster munitions were dropped on Kafr Halab, a village southwest of Syria's second city of Aleppo, on October 4.
"It's disturbing that yet another type of cluster munition is being used in Syria given the harm they cause to civilians for years to come," said Nadim Houry, HRW's deputy Middle East director.
"Neither Russia nor Syria should use cluster munitions, and both should join the international ban without delay."
Russia launched an aerial bombing campaign against opponents of President Bashar al-Assad on September 30.
Cluster munitions contain dozens or hundreds of bomblets and are fired in rockets or dropped from the air.
Widely banned, they spread explosives over large areas and are indiscriminate in nature, often continuing to maim and kill long after the initial attack when previously unexploded bomblets detonate.
Their use in Kafr Halab coincides with the emergence of evidence that they have been used in the provinces of Aleppo, Hama and Idlib since Russia launched its intervention.
HRW said it "cannot conclusively determine whether Russian or Syrian forces were responsible for the attack" on October 4.
It noted that neither country had banned their use.
The watchdog has documented the use of cluster munitions before in the Syrian conflict, with government forces dropping them from the air since 2012 and the Islamic State group firing them from rockets since late 2014.
The conflict, which began as an uprising against Assad's regime in 2011, has escalated into a multi-sided civil war that has drawn thousands of jihadists from overseas.
It is estimated to have killed up to 250,000 people and forced millions to flee the country.