Russia is in the midst of attempts to secure the release of two of its citizens held captive by the Ukrainian government after they were captured in war-torn eastern Ukraine, a Kremlin spokesman confirmed Friday. Moscow continues to deny that the men, identified as Yevgeny Yerofeyev and Alexander Alexandrov, are active members of Russia’s armed forces, despite an independent monitoring group’s claim Thursday that the captives confessed to serving in the Russian army.

“I cannot add anything more to what I have already said. They are Russian nationals who are in captivity. Naturally, the Russian side is taking necessary measures to rescue them from captivity. There is nothing else I can say,” Kremlin spokesman Dmitry Peskov said, according to the Russian news agency TASS.

Ukrainian government forces captured the two men somewhere in eastern Ukraine on May 16. Ukraine vowed to prosecute them for “terrorist acts” unless they confess in open court that they are active members of the Russian army. The Kremlin has repeatedly denied claims by Ukraine, the European Union and the NATO military alliance that Russian soldiers are on the ground in eastern Ukraine, stating only that any Russian nationals in the region are volunteers. More than 6,000 people have died in fighting between government forces and pro-Russian separatist rebels since last year.

“There have never been [Russian] troops in Donbass,” Peskov said, referring to a region in eastern Ukrakine. “We and the Defense Ministry have said many times that there are no Russian military in Donbass.”

Despite Russia’s denial, Ukrainian government representatives have expressed certainty that both Yerofeyev and Alexandrov are both active members of the Russian army. The Organization for Security and Co-operation in Europe, an independent monitoring group that serves as a security watchdog in crisis areas, interviewed the two men at a Kiev hospital Thursday and said they confessed to service in Russia’s armed forces.

“Both individuals claimed that they were members of the unit of the Armed Forces of the Russian Federation. They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack,” the OSCE said in a report obtained by Agence France-Presse.

It’s unclear what specific measures Russia has taken in its bid to secure Yerofeyev and Alexandrov’s release. The two men could face life in prison if convicted on terrorism charges, the Guardian reported.