Accusations of direct Russian support for pro-Russian separatists in Ukraine have prompted Western governments to impose sanctions on Moscow, despite its vehement denials, and fanned tensions with NATO to levels not seen since the Cold War.
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 10 Russian soldiers on Monday.
He 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 Belarussian 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 U.N. -- 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 Moscow would "contribute to create a situation of trust".
But Wednesday's new accusations from Ukraine made clear that the poisonous dispute over Russia's role remained unresolved.
German Foreign Minister Frank-Walter Steinmeier said it was hard to tell whether the talks in Minsk marked a breakthrough.
"Perhaps not, but let's hope that this meeting was not an end of some development, but another beginning," he said.
Chancellor Angela Merkel's spokesman said the flow of Russian forces and weapons into Ukraine was a major problem and Moscow must ensure that it stops.
"It's long overdue that this border is properly secured and that all forms of military support for the separatists over this border end. Russia has a big responsibility for that," spokesman Steffen Seibert said.

Latest News from World News Desk