Ukraine vowed on Monday to present to the public two Russian soldiers it claimed to have captured while fighting Moscow-backed forces in the separatist east.
The politically-charged declaration came as a tenuous February truce was broken by more violence that claimed the lives of at least four Ukrainian servicemen.
Russia firmly denies any involvement in the Ukrainian conflict and accuses the pro-Western leadership in Kiev of waging a war of attrition against its own people in the industrial east of the ex-Soviet state.
But it concedes that some "volunteers" and off-duty soldiers may have crossed Russia's southwestern border to support separatist militias fighting in Ukraine's Lugansk and Donetsk rustbelt.
"For us, it is very important to present to the entire world Russian soldiers who supposedly do not exist on our land," Ukrainian military spokesman Vladyslav Seleznyov told AFP.
"These are fighters from the 3rd non-divisional brigade of the special forces. They are based in (the Volga River city of) Togliatti," he said.
The Ukrainian defence ministry said reporters would be able to see the wounded soldiers at a medical facility in Kiev on either Monday or Tuesday.
Kiev's claim was briskly brushed off by Russian President Vladimir Putin's spokesman Dmitry Peskov.
Both the military and other Moscow authorities have stressed repeatedly that "there are no Russian soldiers in (Ukraine)," Peskov told Moscow Echo radio.
"But I otherwise cannot comment," he added. "In this case, this is something for the experts or our defence ministry to do -- to say how plausible this material is."
- Lugansk gun battle -
Details disclosed by Ukrainian officials since the troops' purported capture on Saturday suggest they were seized in a gun battle between Russian special forces and Ukrainian troops in Lugansk province.
"There were about 14 of them," Seleznyov said in reference to the Russian unit. "We lost one soldier from the 92nd brigade."
Ukraine's Security Service said its investigators were in the process of interrogating the soldiers because both were under suspicion of staging "terrorist acts".
"What will be their fate? They are facing criminal sanctions," the security service said in a statement.
Ukrainian media earlier had identified the servicemen as Aleksander Aleksandrov and Yevgeny Yerofeyev.
The reported capture had been initially claimed by the Aidar Battalion of volunteer fighters that Russian media and the Kremlin portray as vicious neo-Nazi gunmen who commit war crimes.
Aidar on Monday backtracked from its initial claim of responsibility and confirmed that the Ukrainian 92nd brigade was involved.
Kiev's ability to prove the Kremlin's direct involvement in the fighting that has caused 6,250 deaths in the past 13 months would put additional pressure on Moscow to change course on Ukraine.
Russia affirmed its commitment to peace during marathon talks that the leaders of Germany and France held with Putin and Ukrainian President Petro Poroshenko in February.
The ceasefire and political blueprint signed by all sides at the time had briefly helped stem fighting but now seems to be unravelling.
In its latest toll, Kiev said on Monday two more Ukrainian soldiers and the same number of volunteer fighters had been killed in Lugansk province.
Lugansk is the smaller of the two separatist Russian-border regions and had previously been comparatively calm.
But in recent weeks there has been fresh fighting in Lugansk that Kiev blames on a new deployment of Russian troops.