Kiev on Tuesday showed off two purported Russian soldiers it captured during a gun battle with separatist fighters in the industrial east of the war-wrecked state.
The two wounded men were being treated in a Kiev military clinic. One of them shielded his face while speaking briefly to reporters from his hospital bed.
The other man refused to answer a question about whether he was an active member of the Russian armed forces.
Moscow on Monday said both had previously served in the Russian military but had since been demobilised.
Ukraine's media on Tuesday said Captain Yevgeny Yerofeyev and Sargeant Aleksander Aleksandrov were both contract soldiers who had joined the army's main intelligence branch.
"I would like to express thanks to the hospital doctors for their professional help," Yerofeyev said while shielding his face.
He said he would like to tell "my relatives that I am fine, alive and well... Of course, it would be good to keep this from getting in the press."
The Kiev authorities said they would put both on trial for committing acts of terror and return them to Russia if they fully confessed.
The two men wore brown pyjamas and were each recuperating in their own individual rooms. Each was being protected by masked guards.
Reporters were granted access after the men were visited for about 10 minuted by representatives of European Union and the Organization for Security and Co-operation in Europe (OSCE).
Several member of the Amnesty International human rights group also joined the European inspectors' group.
Kiev's ability to prove the Kremlin's direct involvement in fighting that has caused 6,250 deaths in the past 13 months would put additional pressure on Moscow.