Ukraine vowed on Monday to show off 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 Russian defence ministry for its part said the two men were ex-soldiers "who were no longer in the Russian armed forces at the time of their capture".

- US urges fair treatment -

Earlier Monday, Ukrainian army chief Viktor Muzhenko called in reporters for a special briefing designed to showcase what Kiev hopes is irrefutable evidence of Russia's entanglement in one of Europe's bloodiest recent wars.

A fatigues-clad Muzhenko assistant held up a nine-millimetre calibre rifle he said was seized from one of the invading Russians.

Muzhenko added that Ukraine had invited observers from the Organisation for Security and Co-operation in Europe -- a Cold War-era body designed to secure peace across the continent -- to visit the prisoners at their current medical facility in Kiev.

But he appeared to backtrack from an earlier army promise to parade the wounded prisoners before international media crews.

Such a step would almost certainly outrage the Kremlin and potentially violate international treaties on the humane treatment of prisoners of war.

US Assistant Secretary of State Victoria Nuland stressed during a visit to Moscow that Washington welcomed Kiev's decision to invite Red Cross doctors to examine the Russians "in correspondence with the Geneva convention."

Muzhenko identified the two men only as Captain Yevgeny Yerofeyev and Sargeant Aleksander Aleksandrov. Both were allegedly members of a special forces unit that Kiev said was orchestrating recent battles near the Lugansk region town of Schastya.

The Ukrainian army's claim was briskly brushed off by Russian President Vladimir Putin's spokesman Dmitry Peskov.

Both the military and other Moscow authorities have repeatedly pointed out that "there are no Russian soldiers in (Ukraine)," Peskov told Moscow Echo radio.

"But I otherwise cannot comment," he added, referring further questions to the defence ministry.

Defence ministry spokesman Igor Konashenkov said the captured pair were "former Russian soldiers" who were no longer active servicemen.

"These men have indeed done military service in a unit of the Russian army," he was quoted as saying by RIA-Novosti news agency, adding that Russia was counting on the duo "being released as soon as possible".

- Lugansk gun battle -

Details disclosed by Kiev since the troops' purported capture on Saturday suggest they were seized in an extended gun battle between highly-trained Russian special forces and an Ukrainian brigade partially comprised of recent army call-ups.

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.

Kiev's claim that it can prove the Kremlin's direct involvement in fighting that has caused 6,250 deaths in the past 13 months could 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 the fighting but now seems to be unravelling.

Lugansk is the smaller of the two separatist Russian-border regions and had previously been comparatively calm.

But recent weeks have witnessed fresh exchanges of heavy weapons fire in Lugansk that Kiev blames on a new deployment of Russian troops.

A report released last week and based on research done by murdered Russian opposition leader Boris Nemtsov claimed that more than 200 Russian soldiers have been killed fighting in Ukraine.