In a joint report, the Switzerland-based Small Arms Survey and Ukrainian Center for Security Studies (CENSS) said Ukrainian authorities had since the start of the Russian invasion seized substantially more weapons held illegally.

But, they said, "there is little evidence to support claims by the Russian government and pro-Russian media that a large percentage of Western weapons imported by Ukraine are being diverted to armed groups and criminals."

Firearms seizures increased by more than 60 percent between the three years prior to the start of Russia's full-scale invasion of Ukraine and the first three years of the offensive, the report said.

These include handguns and shoulder-fired weapons, including automatic rifles, submachine guns, and light machine guns.

Over the same period, seizures of grenade launchers, grenades, rocket launchers, and mines more than doubled according to the report based on figures from the Ukrainian judicial authorities.

There had been concerns expressed in the early months of the war, including by the then director general of Interpol, that Western arms sent to Ukraine could find their way to criminals in Europe.

These claims have been amplified by Russian officials and also pro-Moscow social media accounts seeking to expose the alleged dangers of the West's military support for Ukraine.

The report also said Russian security forces themselves contribute both directly and indirectly to illicit arms flows in Ukraine.

Russian arms caches are pre-positioned for use in covert operations and while pro-Moscow "proxy forces" are armed in the east.

Meanwhile, the loss and abandonment of Russian weapons on the battlefield and in adjacent areas also contributes to illegal flows of weapons, it said.

"Western hand grenades, shoulder-fired rockets, and portable missiles comprise only a small percentage of all seized weapons," the report said, adding that it found allegations of international trafficking of these arms from Ukraine to be "erroneous, unsubstantiated, or exaggerated".

The report however acknowledged the risk of illicit proliferation and diversion and said as long as the war continues such illegal flows could grow.

"Ukrainian authorities have shown a strong commitment to addressing this issue, but additional training, equipment, and technical resources are required to sustain and expand their ability to disrupt trafficking networks," it said.