DONETSK, Ukraine — For weeks, rumors have flown about the foreign fighters involved in the deepening conflict in Ukraine’s troubled east, each one stranger than the last: mercenaries from an American company, Blackwater; Russian special forces; and even Chechen soldiers of fortune.

Yet there they were on Tuesday afternoon, resting outside a hospital here: Chechen men with automatic rifles, some bearing bloodstained bandages, protecting their wounded comrades in a city hospital after a firefight with the Ukrainian Army.

“We received an invitation to help our brothers,” said one of the fighters in heavily accented Russian. He said he was from Grozny and had fought in the Chechen War that began in 1999. He said he arrived here last week with several dozen men to join a pro-Russian militia group.