The Kremlin said Friday that fighters from Syria and the Middle East would be allowed to fight for Russia in Ukraine, after President Vladimir Putin backed a plan to send volunteers to battle there.

Kremlin spokesman Dmitry Peskov told reporters that Russia's defence minister had said that "most of those who want and who asked (to fight) are citizens of Middle East countries and Syrians".

Russia was a key partner to Syrian President Bashar al-Assad when war broke out, rallying to his side in the conflict in 2015 and turning the tide of the fighting in the regime's favour.

Peskov said the decision to send volunteer fighters to Ukraine was acceptable, claiming that the United States was backing measures to send mercenaries to fight alongside Kyiv's army in Ukraine.

Russia launched a massive military operation in Ukraine late last month, spurring an exodus of refugees in Europe and allegations of war crimes.

"If the West is so enthusiastic about the arrival of mercenaries, then we also have volunteers who want to participate," Peskov told journalists.

Putin backed plans to allow volunteers -- including from abroad -- to fight in Ukraine earlier Friday and Defence Minister Sergei Shoigu said more than 16,000 mostly Middle Eastern volunteer had appealed to join the military action.

Ukraine has announced the creation foreign volunteer contingents to be integrated into its armed forces to fight Russian forces on its territory.