Cluster bombs killed or injured more than 350 people in Syria and Yemen last year and evidence is growing that Russian forces are using the munitions in Syria, a report said on Thursday.
Handicap International, which compiled the study, said there was "compelling evidence" that Russian forces were using cluster munitions in Syria, a claim denied by Moscow.
A total of 248 people were killed or wounded by the munitions in Syria in 2015, the report said, with civilians making up the vast majority of the victims.
Around the world, a total of 417 victims, dead or injured, were recorded in 2015.
The NGO said almost all the victims around the world were civilians and in a third of the cases, children bore the brunt of their devastating effect.
The group urged signatories to the 2008 Oslo Convention on Cluster Munitions to demand that the warring sides to stop using them.
The Oslo signatories are to meet in Geneva next week.
Syria has seen a sharp rise in the use of cluster bombs, with 76 attacks recorded from September 2015 to July this year, the charity said, warning that the real number was probably far higher.
The rise in the number of attacks roughly coincided with the start of Russian military intervention in Syria, it said.
"There is compelling evidence that Russia is using cluster munitions in Syria and/or directly participating with Syrian government forces in attacks using cluster munitions on opposition-held areas of governorates such as Aleppo, Homs and Idlib, and on armed opposition groups," the report said.
Cluster bombs, which consist of canisters that spray bomblets indiscriminately, also killed or maimed 104 people in Yemen.
In Yemen, the Saudi-led military coalition fighting Shia rebels is accused of using the munitions.
Handicap International said that in both Syria and Yemen, a large number of the attacks hit markets, schools and hospitals.
The report highlights the continuing contamination of unexploded cluster bombs in Cambodia, Iraq, Laos and -- half a century after US forces withdrew from the country -- Vietnam.
The United States is not a signatory to the global agreement to limit the use of the munitions.