Putin And Poroshenko Handshake At 'Peace Talks'
"UBS said it had opened a criminal probe after soldiers from the 98th airborne division based in central Russia were detained near the village of Dzerkalne, around 30 miles (50km) from rebel-held Donetsk.
A Ukrainian military spokesman said the men were on a "special mission".
But Russian military sources quoted by state news agencies have claimed the soldiers
crossed the border by mistake.
In footage posted on the official Facebook page of the Ukrainian government's "anti-terrorist operation", the men were shown dressed in camouflage fatigues.
One of them, who identified himself as Ivan Melchyakov, listed his personal details, including the name of the paratroop regiment he said is based in the Russian town of Kostroma.
"I did not see where we crossed the border. They just told us we were going on a 70-kilometre march over three days," he said.
"Everything is different here, not like they show it on television. We've come as cannon fodder," he said in the video.
Ukraine's Defence Minister Valeriy Geletey said the soldiers were captured on Monday.
"Officially they are at exercises in various corners of Russia.
"In reality, they are participating in military aggression against Ukraine and their families know nothing about their true fate.
"I am addressing the relatives of Russian servicemen: find out immediately where your loved ones are. Take them out of Ukraine, where they are being forced to die."