Ukraine's border-guards service says it has detained a Russian officer who was driving in a military truck packed with ammunition in the country's war-torn east.
The service said in a statement that the man acknowledged he was a Russian major in a rocket-artillery unit.
"He had no documents," said border guards spokesman Oleksandr Tomchyshyn. "He is responsible for ammunition supply. He said that while delivering the ammunition they had got lost."
Another man also detained late on July 25 in the truck identified himself as a pro-Russian separatist fighter.
The two men reportedly wore military uniforms without insignia.
The border-guards service said it found nearly 200 cases containing grenades and ammunition, including rocket-propelled shells, in the truck.
The vehicle was stopped about 45 kilometers southwest of Donetsk, the largest city in eastern Ukraine under rebel control.
It was reportedly driving from the direction of Olenivka, a town also held by the separatists, and halted only after Ukrainian border guards fired warning shots.
"We can assume that they took a wrong direction while driving, got lost and came on our checkpoint," military spokesman Oleksandr Motuzyanuk told journalists.
There was no immediate comment from the Russian military.
The self-proclaimed defense ministry of the rebel forces in Donetsk rejected Ukraine's claim, saying it "provokes irony."
Ukrainian state security agents have already questioned the alleged Russian major.
If he is confirmed to be a Russian soldier, his detention will lend weight to Ukraine's charges that Russia is directly backing the separatist insurgency in eastern Ukraine and failing to honor a peace agreement signed in Minsk, Belarus, in February.
Moscow denies its regular forces are engaged in the conflict on behalf of the separatists, despite what Kyiv and Western governments say is undeniable proof.
According to the United Nations, the conflict has killed more than 6,400 people since erupting in April 2014 in Ukraine's industrialized, Russian-speaking east.