Ukraine's president has told the BBC his country is now in a "real war" with Russia - and that Ukrainians should prepare for a Russian offensive.
Petro Poroshenko told the BBC's Fergal Keane he did not trust his Russian counterpart, Vladimir Putin, but had no option but to negotiate with him.
The Kremlin said it did not trust Mr Poroshenko either and accused Ukraine of "broken commitments".
Meanwhile Ukraine has charged two men it accuses of being Russian soldiers.
Russia denies Western accusations that it has sent regular troops and armour to help rebels in eastern Ukraine.
The United Nations says at least 6,000 people have been killed since fighting started in eastern Ukraine in April 2014.
'We should be ready'
Mr Poroshenko said he believed Russia was preparing for "an offensive" in the summer.
"I think we should be ready and I think that we do not give them any tiny chance for provocation. That will totally be their responsibility," he said.
He said it was possible parts of eastern Ukraine could not be taken away from rebels by military means alone - meaning diplomacy was the only answer.
Kremlin spokesman Dmitri Peskov responded by saying there was a "serious lack of trust" between the two countries, which stemmed from "specific broken commitments and... failure to implement the provisions of signed documents".
Speaking in Russia's upper chamber of parliament on Wednesday, Foreign Minister Sergei Lavrov condemned "radicals in Kiev and some other capitals" for trying to provoke a "further escalation" in the conflict. He warned this would have "terrible consequences both for Ukraine and the entire European security system".
Pro-Russian rebels have made significant gains in eastern Ukraine, including, most recently, the rail hub of Debaltseve. They, and Russia, have denied they are receiving Moscow's support.
But Russia's role has again come into question in the past week, after the capture on Saturday of two men who Ukraine said were elite Russian soldiers in the Luhansk region.
In video statements posted by Ukraine's security service, Aleksandr Aleksandrov and Yevgeny Yerofeyev confirmed they were active Russian servicemen. However, Moscow said they were not serving "at the moment they were detained".
They were charged with involvement in "terrorist activity" on Wednesday, according a Ukrainian security official.
Mr Poroshenko said: "Can I be absolutely clear with you, this is not a fight with Russian-backed separatists, this is a real war with Russia.
"The fact that we captured... Russian regular special forces soldiers [is] strong evidence of that."
On Tuesday, Nato Secretary General Jens Stoltenberg called for an end to snap exercises by Russia close to the Ukrainian border, and demanded Russia be "more transparent when it comes to their military activities".
A ceasefire agreement was signed in Minsk in February, but heavy fighting has continued since then. Last week, Mr Poroshenko said at least 83 Ukrainian servicemen had been killed since the ceasefire came into effect.
Elsewhere in Ukraine, at least four Ukrainian soldiers were killed in the eastern Luhansk region on Tuesday. The region's governor said the men were attacked by separatist rebels near the village of Katerinovka.