Russian President Vladimir Putin has ordered so-called volunteer fighters from the Middle East to be brought into Ukraine.
Defence Minister Sergei Shoigu said Russia knew of “more than 16,000 applications”, according to a statement from the Kremlin.
Most "volunteers" are people who had been helped by Russia against the so-called Islamic State (IS) group, Shoigu said.
They want “to take part in what they consider a liberation movement”, Shoigu added.
Since 2015, Russian forces have backed Syrian President Bashar al-Assad against various groups opposed to his rule, including IS.
Syria has been one of the very few countries to openly support Russia's invasion of Ukraine from the outset.
Putin told Shoigu that Russia should help would-be volunteers to “move to the combat zone” alongside Russian-backed forces in the breakaway Donbas region of eastern Ukraine.
"If you see that there are these people who want of their own accord, not for money, to come to help the people living in Donbas, then we need to give them what they want," Putin said at a Russian security council meeting.
Moscow has accused the West of recruiting foreign "mercenaries" to fight for Ukraine.
Ukrainian President Volodymyr Zelenskyy recently stated that 16,000 foreigners had volunteered for an "international legion" to fight for the Kyiv government.