Kiev said its forces were battling an armoured column that crossed from Russia on Monday, as Moscow unveiled plans for a new aid convoy for east Ukraine, ramping up tensions on the eve of crunch talks on the conflict.
After a string of setbacks, separatist fighters appeared to have launched a counter-offensive across the region just as Ukrainian President Petro Poroshenko and Russian leader Vladimir Putin geared up to meet for the first time in months alongside top EU officials in Minsk on Tuesday.
Poroshenko expressed his "extreme concern about the breaching of Ukraine's border by Russian armoured vehicles and Russia's intention to send a so-called 'humanitarian' convoy again" in a phone call with EU Council President Herman Van Rompuy.
Ukrainian military spokesman Andriy Lysenko said troops had halted a column of a dozen tanks and armoured personal carriers flying rebel flags after it crossed from Russia heading towards the government-held port city of Mariupol.
Lysenko said the column was manned by Russian soldiers "disguised" as local rebels.
If confirmed, the incursion could represent a dramatic attempt by the insurgents to push into government-controlled territory.
But Russian Foreign Minister Sergei Lavrov dismissed the claims, saying "there has been more than enough disinformation about our invasion. No doubt some foreign newspaper will print that 'news' tomorrow."
Lysenko said the insurgents had waged "unsuccessful counter-attacks" in eight towns south of Donetsk over the past 24 hours, after a rebel supremo had warned of a strike back against Kiev's advances.
Explosions ring out
An AFP journalist witnessed heavy fighting to the south of Donetsk, the main rebel bastion in eastern Ukraine, where separatists said they had deployed fresh tanks and artillery.
Explosions rang out and smoke rose from towns to the south of the city. Ukraine's military said four soldiers had been killed and 31 wounded in the past 24 hours.
The Kremlin also ratcheted up the pressure by announcing plans to send another aid convoy into eastern Ukraine "this week".
Russia unilaterally sent about 230 lorries carrying what it claimed was 1,800 tonnes of humanitarian aid to the rebel-held city of Lugansk on Friday after accusing Kiev of intentionally delaying the mission.
Kiev condemned the move as a "direct invasion" and the West accused the Kremlin of a dangerous escalation in the four-month conflict.
But the trucks returned to Russia on Saturday apparently without incident.
Over 2,200 people have died and 400,000 fled their homes since April in fighting that has left residents in some besieged rebel-held cities without water or power for weeks.
"We would like to agree on the conditions to send the convoy on the same route with the same participation of Ukrainian border guards and customs officers as soon as possible," Lavrov told a news conference in Moscow.
Putin and Poroshenko are to meet in a new push to try to end a crisis that erupted in eastern Ukraine after Russia unilaterally annexed Crimea in March.
"Everyone suddenly realised that the continuation of the conflict may lead to a full-scale catastrophe," said editor of Russia in Global Affairs, Fyodor Lukyanov.
Poroshenko has pledged to "talk peace" with Putin but insists the withdrawal of pro-Kremlin forces is the only way to end the conflict threatening to tear apart the country on the EU's eastern flank.
Kiev has accused Russia of boosting arms supplies to the rebels as government troops cut deeper into their territory. Moscow has retorted that Ukrainian forces must cease their punishing offensive.
Lavrov played down hopes for a major breakthrough in Minsk, saying only that the talks would "facilitate the exchange of opinions" about "efforts to start the political process to settle the political crisis".
Both sides are under pressure from the international community to find a compromise.
German Chancellor Angela Merkel called for a bilateral ceasefire and tighter border controls during a weekend visit to Kiev while stressing support for Ukraine's territorial integrity.
She said she wanted to find a way out of the crisis "that doesn't harm Russia", with the EU and US already having slapped the harshest economic sanctions on Moscow since the collapse of the Soviet Union.
Passions rose further after rebels on Sunday paraded dozens of captured soldiers past an angry crowd in Donetsk in an event timed to undermine a military parade taking place in Kiev to mark Ukraine's Independence Day.
Kiev's defence minister called the parade "a challenge not just to Ukrainian society but to the world."
Human Rights Watch said it was "a blatant violation of the laws of war".
But Lavrov said he "didn't see anything close to what could be considered as humiliating" in images of the event.