DONETSK, Ukraine – His Adidas sneakers betrayed him. His camouflage clothes differed from the usual dark green ones that Kremlin-backed fighters wear in separatist-held territory.His name is Yuriy, a Russian claiming to be a volunteer. He has just arrived in Donetsk, the separatist-controlled provincial capital, on a bus from Rostov-on-Don in Russia.

He reacts angrily to questions about why he is in Ukraine fighting.