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This would include thousands of Syrian veteran volunteers who wanted to join the war, Defence Minister Sergei Shoigu said on Friday, a plan with the backing of President Vladimir Putin.
The comments come days after US officials told The Wall Street Journal that Russia was recruiting fighters from Syria, where it has a sizeable military presence to bolster the forces backing President Bashar Al Assad.
Footage shared by Russia’s Defence Ministry showed dozens of men in camouflage, hoisting Kalashnikov assault rifles and pro-Russian banners who it said were Syria veterans eager to join the Ukraine conflict.
Moscow's troops are edging closer to the capital Kyiv, two weeks after Mr Putin announced a "special military operation" in Russia's pro-western neighbour.
It is unclear if the volunteers would be integrated into the regular Russian military, if Moscow will establish brigades or if those signing up will join Ukrainian separatist forces that are fighting alongside Russia.
"If you see that there are people who want on a voluntary basis [to help east Ukraine's separatists], then you need to meet them halfway and help them move to combat zones," the president told Mr Shoigu during a televised security council meeting.
Kremlin spokesman Dmitry Peskov told reporters that the Defence Ministry had "talked in particular about those who sent their requests from the Middle East countries and from Syria".
"There was no talk about our fellow citizens [volunteering],” he said.
On the opposing side, tens of thousands of volunteers have signed up to join Ukrainian forces to push back the Russian invasion.
Mr Peskov said the decision to send volunteer fighters to Ukraine was within reason, claiming that the US was backing measures to send mercenaries to join Kyiv's army.
"If the West is so enthusiastic about the arrival of mercenaries, then we also have volunteers who want to participate," Mr Peskov said.
Russian mercenaries gave vital support to pro-Moscow separatists in eastern Ukraine in 2014, observers say, when the Kremlin annexed the Crimean peninsula after street rallies ousted a Kremlin-friendly leader.
There are also sizeable numbers of Russian mercenaries alongside its military deployment since Syria 2015, when Moscow’s heavy air campaign decimated the opposition and turned the tide in favour of Assad.
Russia launched its offensive in Ukraine late last month, spurring an exodus of refugees to Europe and allegations of war crimes.
Members of Nato, the US-led military alliance that Ukraine seeks to join, have sharply increased weapons supplies to Ukraine, and are bolstering troop numbers near Russia.
Mr Putin on Friday asked Mr Shoigu to prepare plans for the possible fortification of Russia's western border "in response to actions taken by Nato countries".
Poland and the three Baltic states share a common border with Russia, while Ukraine borders Nato members Hungary, Romania and Slovakia.
Mr Putin said any weapons seized during the fighting by Russian troops – particularly arms manufactured by the West – should be handed to rebels in eastern Ukraine, regions Moscow recognised as independent shortly before the invasion.
"I support the possibility of transferring them to the military units of the DNR and LNR," Mr Putin said of confiscated weapons, referring to the so-called Donetsk and Luhansk People's Republic, respectively.
Ukraine's army had been fighting the rebels since 2014, before Russia's incursion, in fighting that has claimed 14,000 lives.