Ukraine accused Russia of launching a new military incursion across its eastern border this evening.
Hopes are fading that talks yesterday between the country's presidents might mark a turning point in a five-month-old crisis.
Accusations of direct Russian support for pro-Russian separatists in Ukraine have prompted Western governments to impose sanctions on Russia, despite its denials.
Ukrainian military spokesman Andriy Lysenko said a group of Russian soldiers had crossed the border in armoured infantry carriers and a truck and entered the town of Amvrosiyivka, not far from where Ukraine detained ten Russian soldiers on Monday.
Ukraine's Security Services also said in a statement it had detained another Russian soldier in the east of the country who has confessed his unit provided military support to separatist rebels.
Mr Lysenko said fighting in two other towns, Horlivka and Ilovaysk, had killed about 200 pro-Russian rebels and destroyed tanks and missile systems.
Thirteen Ukrainian service personnel had been killed in the past 24 hours and 36 had been wounded.
No comment was immediately available from the Russian defence ministry on the alleged incursion.
Russia denies sending weapons and soldiers to help the rebels, and says the men captured on Monday had crossed an unmarked section of the border by mistake.
Late-night talks in the Belarusian capital Minsk had appeared to yield some progress towards ending a war in which more than 2,200 people have been killed, according to the UN, a toll that excludes the 298 who died when a Malaysian airliner was shot down over rebel-held territory in July.
Ukrainian President Petro Poroshenko said he would work on an urgent 'road map' towards a ceasefire with the rebels.
Russia's Vladimir Putin said it would be for Ukrainians to work out ceasefire terms, but Russia would "contribute to create a situation of trust".
But today’s new accusations from Ukraine made clear that the poisonous dispute over Russia's role remained unresolved.
In a telephone call with Mr Putin, Germany Chancellor Angela Merkel said reports of a new Russian military incursion into Ukrainian territory had to be cleared up.
"The latest reports of the presence of Russian soldiers on Ukrainian territory must be explained," Ms Merkel's spokesman Steffen Seibert said.
"She emphasised Russia's major responsibility for de-escalation and watching over its own frontiers."
A senior NATO diplomat said Russian support for the separatists was becoming increasingly open.
"I think there's a shift here that we may be witnessing, very recently, from largely covert, ambiguous, deniable support to what appears increasingly to be flat-out, overt and obvious [support] and with the only form of ambiguity being that the Russians ... claim it is not happening," said the diplomat, speaking to reporters in Brussels on condition of anonymity.
He said increasingly sophisticated weapons systems were now in the area, including the SA-22 surface-to-air missile, which is more advanced than the SA-11 system that many Western officials suspect rebels used to bring down the Malaysian jet.
The United States said today new military incursions on Ukraine's eastern border indicate a Russian-directed counter-offensive is likely under way in two Ukrainian towns.
White House spokesman Josh Earnest said the military movement "would be consistent with the other kinds of destabilising military activities that Russia has pursued in Ukraine".
Ukraine's State Security services said in a statement that it had detained a private in Ukraine's easternmost Luhansk Province from a unit stationed in Russia's Rostov region near the border with Ukraine.
The statement said that when questioned, he said his military unit transfers military equipment and ammunition including multiple rocket launchers BM-21 Grads as well as armoured personnel carriers to the rebels.
Fighting in the east erupted in April, a month after Russia annexed Ukraine's Crimean peninsula in response to the toppling of a pro-Moscow president in Kiev.
The crisis has prompted the United States and EU to impose sanctions on Russia's finance, oil and defence sectors, and Russia has hit back by banning most western food imports.
The trade wars threaten to tip Russia into recession and hamper economic recovery in Europe.