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Up to 20,000 from the Russian private military company the as well as from Syria and Libya are fighting alongside Moscow's forces in , a European official said Tuesday.

"Regarding their capabilities, it's infantry. They don't have any heavy vehicles and weapons. It's much more infantry," the official told reporters in Washington, speaking on condition of anonymity.

"These guys are mainly used as a mass against Ukrainian resistance," the official said.

He added that of the estimated 10,000 to 20,000 mercenaries taking part in the invasion, it is difficult to say how many are from the Wagner Group, which is said to be close to Russian President Vladimir Putin, and how many are from Syria and Libya.

The official said there has been some "transfer" of fighters from Libya and Syria to the eastern Donbas region of Ukraine.

In late March, the British defence ministry said more than 1,000 mercenaries from the Wagner Group would likely take part in fighting in Ukraine, including senior leaders of the organization.

The European official said that in four to six months the war will probably be in a stalemate, with Russian troops controlling Lugansk, which is part of Donbas, and a tiny land bridge within Zaporizhzhia province in southern Ukraine.

The official also predicted that by the autumn of this year negotiations between Russia and Ukraine could conclude, which would bring the war to an end before winter sets in.