Ukraine in particular has a history as a hub of the arms trade during the 1990s, setting off alarm bells for those who study illicit flows.

"There are very significant risks associated to the proliferation of weapons in Ukraine at the moment, in particular regarding small arms and light weapons," said Nils Duquet, a researcher and director of the Flemish Peace Institute.

Western nations, above all the US, have announced successive shipments of both light and heavy weapons for Kyiv's forces since Russian troops crossed the Ukrainian border on February 24.

Washington alone has delivered or promised military gear including hundreds of Switchblade kamikaze drones, 7,000 assault rifles with 50 million rounds of ammunition, laser-guided missiles and radar systems to detect enemy drones and incoming artillery fire.

"While the response to provide more weapons to Ukraine is understandable..., it would be prudent to consider the immediate and long-term security implications," the US-based Stimson Center think-tank said in March.

"We've seen time and time again how arms aimed at aiding an ally in one conflict have found their way to the frontlines of unforeseen battlefields, often in the hands of groups at odds with US interests or those of civilians," it added, pointing especially to small arms.

- 'Overwhelming challenge' -

Following the collapse of the Soviet Union, many weapons stockpiled in Ukraine were shipped off to other countries and conflict zones around the world.

That looting of the country's arms supplies resumed following Russia's 2014 annexation of the Ukrainian Black Sea peninsula of Crimea and the seizure of two regions in the east of Ukraine by pro-Moscow separatists.

According to the Small Arms Survey (SAS) by the Geneva-based Institute of International and Development Studies, 300,000 light weapons were stolen or lost between 2013 and 2015, with just 4,000 retrieved since then.

Rather than being shipped abroad as in the 1990s, most of these ended up on the black market within Ukraine, the SAS found.

"The unresolved conflict in the eastern part of the country and general anxiety towards local security conditions" could explain increased demand for weapons among ordinary people, SAS researcher Matt Schroeder said.

Looking ahead, "collecting these weapons and disposing of them properly would be an overwhelming challenge for any government, let alone one that is still fighting an existential threat," he added.

- 'Bank robberies with Javelins' -

Even before the current conflict, the US military's inspector general in 2020 questioned the level of surveillance of weapons sent to Ukraine.

Annie Shiel of the Center for Civilians in Conflict (Civic) warned that "there has been very little transparency around what risk mitigation or monitoring steps the US and other countries sending weapons to Ukraine have taken, if any... to ensure the protection of civilians".

The aid group has called for deliveries to be tied to human rights commitments and for the arms to be tracked after they are handed over.

Other experts see the task of following arms through conflict zones as all but impossible.

"It's an illusion to think that in a context of war you can actually have control of weapons there. We know that many weapons will not return to the official forces but they will remain in the region for many years," said Nils Duquet.

"Look at Yugoslavia, success has been made but these weapons are still being smuggled in all parts of Europe," he added, predicting similar outcomes for Ukraine.

One senior French military officer evoked a lurid possible outcome.

"We'll be laughing on the other side of our faces once we're seeing bank robberies with Javelins," he said, referring to the US-made anti-tank missiles.

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SAS