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.

Another Ukrainian defence spokesman told reporters that the wounded soldiers had been treated in hospital and were being flown to Kiev for their public appearance on Tuesday.
Kiev's claim was dismissed by Russian President Vladimir Putin's spokesman Dmitry Peskov.

Both the military and other Moscow authorities have repeatedly stressed that "there are no Russian soldiers in the Donbass," Peskov said, referring to eastern Ukraine.

"But I otherwise cannot comment," Peskov told Moscow Echo radio. "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 claimed capture of the soldiers 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."

The 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.
If Kiev can prove the Kremlin's direct involvement in the fighting that has caused  6,250 deaths in the past 13 months, it would put additional pressure on Moscow to change course on Ukraine.

Russia affirmed its commitment to peace during talks that the leaders of Germany and France held with Putin and Ukrainian President Petro Poroshenko in February.

The ceasefire and political blueprint that was signed by all sides at the time had briefly helped stemmed fighting but now seems to be unravelling.

Ukraine's latest toll reported on Monday said 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 remain comparatively calm.

But recent weeks have witnessed more Lugansk fighting that Kiev blames on a new deployment of Russian troops.