Ukraine said Monday that it will prosecute two alleged Russian soldiers it captured fighting in eastern Ukraine. The two men, who were held after being wounded, were detained in the town of Shchastya, which is close to the Russian border.

Vitaliy Naida, Ukraine’s security chief, said there is conclusive evidence linking them to GRU, Russia’s foreign military intelligence branch. "We have battle dress uniform, we have automatic rifles that are produced only in the Russian Federation only for Special Forces' use," Naida told BBC. "We have already started a criminal case for a terrorist act."

Naida added that the two were part of a group of 220 GRU members deployed to the Luhansk region in early May.

One of the two men admitted on video on Monday that he was part of a Russian special forces espionage mission. He reportedly identified himself as Alexander Alexandrov.

Russia said the two men were not acting as Russian servicemen at the time of their arrest and demanded their release. “The citizens of Russia, Alexander Alexandrov and Yevgeniy Yerofeyev, captured by the SBU (Ukrainian Security Service) in Luhansk region, at the time of their arrest on May 17 were not servicemen of the Russian armed forces," Maj. Gen. Igor Konashenkov said in a statement, according to Ukraine Today.

"We checked the information of the Ukrainian side: these guys had previously served in one of the units of the Russian armed forces and had a military training,” he added.

Ukraine’s Foreign Minister Pavlo Klimkin also told reporters on Monday that Russian agents had tried to kill the two men after they had been captured, but declined to go into detail, BBC reported.

Kiev has maintained that Russia is directly supporting the conflict in eastern Ukraine with troops and supplies, but Russian President Vladimir Putin and other Moscow officials have consistently denied the accusations, while admitting that independent Russian volunteers could be fighting there.

Russian troops have been allegedly mobilized in force twice -- once in August in eastern Ukraine's Donbass region, and again during January and February when Russian tank units led the capture of the city of Debaltseve, according to media reports.

A ceasefire has been in place since February, although it has been frequently violated.