KIEV, May 18 - Ukraine vowed on Monday to show off two Russian soldiers it claimed to have captured while fighting Moscow-backed forces in the separatist east, AFP reported.

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 Luhansk 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 defense 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.”

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-millimeter caliber rifle he said was seized from one of the invading Russians.

Muzhenko added that Ukraine had invited observers from the Organization 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 Luhansk 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.

Defense 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.”

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. (afp/ez)