Ukraine said Monday that two Russian servicemen captured in the country's eastern conflict zone would be prosecuted for "terrorist acts," a move that will likely worsen the already strained relationship between the countries.

"They are facing criminal charges, investigators are working with them," Security Service of Ukraine Chief Valentyn Nalyvaichenko said of the Russians during a briefing Monday. Under Ukrainian law, the men face life sentences if found guilty of committing a terrorist act.

The Ukrainian military said documents and information obtained during interrogations proved the two men are Captain Yevgeny Yerofeyev, 28, and Sergeant Alexander Alexandrov, 26, Russian citizens and members of the 3rd Special Forces Brigade of the Armed Forces of the Russian Federation, part of the country's Main Intelligence Directorate, a foreign military intelligence division commonly known as the GRU.

At a televised press conference in Kiev, the commander of Ukraine's 92nd Brigade, Viktor Nikolyuk, said Yerofeyev and Alexandrov were wounded and later captured during a fight between their unit and his forces Saturday afternoon near the town of Shchastya. One Ukrainian soldier was killed and another wounded in the skirmish, he added, showing a rifle allegedly used by the Russians.

Ukrainian commander claims pro-Russian forces opened fire on his men as they carried captured Russians off the field pic.twitter.com/JfPtHvktN2 — Alec Luhn (@ASLuhn) May 18, 2015

The strategic town of Shchastya sits 12 miles north of the separatist-controlled regional capital Luhansk and has changed hands several times over the course of the conflict.

In an interrogation video published online by the Interior Ministry of Ukraine, Alexandrov, resting in a hospital bed after surgery to mend his shattered femur, said he was from the central Russian town of Togliatti and that he was part of a 14-man "dirty tricks" unit tasked with spying on the Ukrainians.

Вчера, 16.05.15., Спецназ РФ атаковал из оккупированного Луганска украинский г.Счастье. Со слов двух раненых пленных рос... Posted by Grigory Maximets on Sunday, May 17, 2015

Alexsandrov also said he had been in the Russian military "starting from Dec. 12, 2012" and that he is currently "a contract soldier." He named his Russian commander as Evgeniy Erofeev.

He added that he been based in Luhansk since March 6; he and his Russian comrades had been rotating in and out of the territory around Shchastya every four to five days. He arrived in Luhansk with his unit in a military "Ural truck."

Euromaidan Press, an independent pro-Ukrainian media platform, published a version of the interrogation video with English subtitles, which can be viewed below.

Yerofeyev was also wounded before being captured, suffering a shoulder injury. Both men were treated in a Shchastya hospital before being transported to Kiev.

An Aidar battalion fighter told the Associated Press the Russians were worried that the battalion's doctors in Shchastya wanted to take their organs for sale.

"They asked not to be sedated because they were afraid we would take their organs," Grigory Maksimets, an Aidar medic told the news service, adding their Russian commanders had warned them about this.

Russia has repeatedly denied playing a direct role in the conflict, despite mounting evidence otherwise. Kiev and western governments say the Kremlin has supplied the separatists with troops, military trainers, weapons and financial support.

This isn't the first time Russian soldiers have been detained in eastern Ukraine. A group of 10 servicemen were taken prisoner last August after they stumbled into Ukraine "by accident," they said. Moscow remarked that the group had simply gotten "lost" and slipped across the border. In the past, the Kremlin has dismissed reports of active Russian servicemen in Ukraine as "volunteers on holiday."

Tanks take part in the rehearsal for the Victory Day military parade which will took place on May 9, in Donetsk, Ukraine, Tuesday, May 5, 2015.

Andriy Lysenko, a Ukrainian military spokesman, told journalists on Monday that the capture of the men will no longer allow Russia to deny its military presence in Ukraine.

"The leadership of the Russian Federation will have difficulty saying that these guys just got lost," he said. "They were there on a mission and they were killing our people."

But the Kremlin again denied the presence of Russian forces in Ukraine.

Dmitry Peskov, spokesperson for Russian President Vladimir Putin, told Mashable the two alleged soldiers captured couldn't possibly be active Russian servicemen. "There has not ever been Russian troops on the ground in Ukraine," he said.

As for the documents and rifle presented by Kiev as evidence of the men being Russian soldiers, Peskov said that "Russian documents are easy to fake" and weapons made in Russia are being used "by both sides."

Major General Igor Konashenkov, spokesman for Russia's Foreign Ministry, admitted Alexandrov and Yerofeyev were in fact government soldiers, but said they "were not active servicemen of the Russian Armed Forces at the time of their apprehension on May 17," the Interfax news service reported.

He urged Ukraine to release the men immediately, and he might have some support for that in Kiev.

Rather than prosecuting the two detainees, Anton Heraschenko, an adviser to Ukraine's interior minister, suggested exchanging the two detained Russian servicemen for pilot Nadia Savchenko, who is currently sitting in a Russian jail cell, and some 400 other Ukrainian prisoners of war.

The 13-month conflict between Russia-backed separatists and Ukrainian forces in easter Ukraine has killed more than 6,100 people and displaced over a million, according to the United Nations. Despite a cease-fire brokered in February that has lessened the intensity of the fighting, battles have continued in areas around the contentious Donetsk airport and the strategic port city of Mariupol.

Kiev says more than 80 Ukrainian soldiers have been killed since the truce. At least four died just last weekend in eastern Ukraine.