The presidents of Russia and Ukraine hold key talks Tuesday with little hopes for a breakthrough in the conflict pitting the former Soviet state against pro-Russian separatist rebels.
Kiev ratcheted up tensions by announcing hours ahead of the talks it had captured 10 Russian soldiers on its territory, while Washington accused Moscow of escalating the conflict with repeat military incursions.
Ukrainian President Petro Poroshenko and Russian leader Vladimir Putin are due to meet in Minsk alongside top EU officials and the leaders from Kazakhstan and Belarus, in a bid to defuse tensions that some fear could trigger direct hostilities between Russia and Ukraine.
US National Security Adviser Susan Rice slammed Moscow for "military incursions" into Ukraine using artillery, air defence systems, tanks and troops, representing a "significant escalation" in the conflict.
"Repeated Russian incursions into Ukraine unacceptable. Dangerous and inflammatory," she wrote on Twitter.
Kiev's security service said paratroopers from Russia's 98th airborne division were captured about 50 kilometres (30 miles) southeast of the main rebel stronghold of Donetsk.
Ukrainian media on Tuesday aired footage purporting to show the captured Russian paratroopers confessing to entering Ukraine in armoured convoys.
"We travelled here in columns not along the roads but across the fields," says one of the men dressed in camouflage.
Kiev has long accused Moscow of stoking the separatist insurgency raging in its east, but this is the first time Ukrainian authorities have claimed to have captured soldiers from Russia's regular army.
"Officially, they are at exercises in various corners of Russia. In reality, they are participating in military aggression against Ukraine", Defence Minister Valeriy Geletey said on his Facebook page.
Moscow has repeatedly denied any involvement in the rebellion in Ukraine and demands Kiev halt its punishing offensive.
A Russian defence ministry source described the captured soldiers Tuesday as having crossed into Ukraine "by accident".
The soldiers had been "taking part in patrolling a section of the Russian-Ukrainian border; they crossed it most likely by accident, on an unequipped, unmarked section", Russian news agencies quoted the source as saying.
Foreign Minister Sergei Lavrov reacted earlier saying suggesting it was more "disinformation" about Russian activity.
On the ground there appeared no end in sight to the four months of conflict that has already claimed some 2,200 lives.
Local authorities in the main rebel bastion of Donetsk said three civilians were killed in shelling overnight as the army pummels insurgent fighters hunkered down there.
An AFP journalist reported the sound of heavy artillery fire to the west and south of Donetsk and saw Grad rockets fired by Ukrainian forces towards the town of Olenivka.
Fighting has intensified in the run-up to the key talks in Minsk with the rebels appearing to launch a counteroffensive after losing swathes of territory to a push by government forces.
It was unclear whether Poroshenko and Putin would hold bilateral talks during the meeting in Minsk, the capital of Belarus.
The two met briefly in France at ceremonies to mark the 70th anniversary of the D-Day landings at the beginning of June.
Officials from the EU and Russian-led Customs Union were set to discuss the crisis as well as trade following the signing of key political and economic agreements by Ukraine's new pro-Western leaders with the European Union in June.
It was the refusal by former president Viktor Yanukovych to ink the EU deal last year, instead choosing to favour Moscow's economic bloc, that sparked the protests that eventually led to his ouster and set in motion a chain of events leading to Russia's annexation of Crimea and the pro-Moscow insurgency in the east.
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".
Some 400,000 people have fled their homes since April in fighting that has left residents in some besieged rebel-held cities without water or power for weeks.