Ukrainian President Petro Poroshenko has said his country was fighting a "real war" with Russia that could escalate at any time.
He said the weekend capture of two purported Russian special service soldiers proved that the separatist uprising in the east of Ukraine was a guise for a Moscow-orchestrated campaigned aimed at breaking up the country.
"Can I be absolutely clear with you this is not a fight with Russian-backed separatists, this is a real war with Russia," the Ukrainian leader told the BBC.
"The fact that we captured... Russian regular special forces soldiers (is) strong evidence of that."
Ukraine's military yesterday showed off two Russians taken prisoner during a firefight in Luhansk - a region that together with neighbouring Donetsk revolted against Kiev's shift towards the West 13 months ago.
The men testified during a taped interrogation that they were members of a 200-strong Russian reconnaissance unit that entered the area nearly two months ago.
The United Nations believes that the entire conflict has claimed at least 6,250 lives and driven more than a million people from their homes.
A second truce agreement Mr Poroshenko struck with Russian President Vladimir Putin with the help of the leaders of Germany and France in February has thus far failed to take complete hold.
Ukraine has lost at least eight servicemen since Monday in clashes across both provinces. Ukrainian armed forces blame the violence on a new infusion of Russian troops.
"I believe they are preparing an offensive and I think we should be ready and... not give them any chance for a provocation. That will totally be their responsibility," the Ukrainian president said.
Mr Poroshenko stressed that he did not necessarily "trust" Mr Putin but had no choice but to seek a diplomatic solution to the crisis with the Russian leader because the war could not be resolved with guns and tanks.
