A military spokesman says Ukrainian soldiers on January 20 came under attack from Russian regular forces in the north of the conflict zone in eastern Ukraine.
Spokesman Andriy Lysenko said at a special news briefing in Kyiv, "In violation of all prior agreements, Ukrainian military units were attacked in the north [of the war zone] by regular units of the Russian armed forces."
The Ukrainian accusation came hours after Moscow denied claims by Kyiv of about 700 new Russian soldiers crossing over into eastern Ukraine amid intensified fighting between Ukrainian forces and pro-Russian rebels.
Lysenko said the attack happened in the Luhansk province -- the region the Russian forces had allegedly entered the day before.
There was no immediate comment from Russia.
The claims are likely to add to tensions surrounding talks in Berlin on January 21 between foreign ministers from Germany, Ukraine, Russia, and France.
German Foreign Minister Frank-Walter Steinmeier, who announced the talks, said in a statement, "The chief aim now is to prevent a further deterioration of the military conflict and a renewed political escalation between Kyiv and Moscow."
He will host his Russian counterpart Sergei Lavrov, Ukraine's Pavlo Klimkin, and Laurent Fabius of France. The group last met in Berlin on January 12.
Fighting intensified in recent days as Ukrainian government troops and pro-Russian separatist fighters have been battling for control of Donetsk airport in eastern Ukraine.
The Ukrainian military claimed its troops have retaken almost all the areas of the ruined airport lost to separatists, but also admitted that the rebels had captured eight of its troops.
"There was a battle. There were deadly losses. And eight people were captured," Ukrainian President Petro Poroshenko's defense adviser Yuriy Biruykov wrote on Facebook.
Russian state television aired footage of the captured men's interrogation on its evening news shows.
Separatist leader, Aleksandr Zakharchenko, said that "all the Ukrainian Army's attempts to take the airport...have failed."
Artillery exchanges could be heard all night across the city of Donetsk -- the rebels' main stronghold.
Donetsk separatist administration official Ivan Prikhodko said two civilians were killed and eight seriously wounded when a shell hit a bus stop on the northwestern edge of town.
Klimkin said on January 20 that pro-Russian separatists had seized more than 500 square kilometers of territory from Ukrainian forces since a cease-fire agreement was signed in Minsk in September.
He said at a news conference in Kyiv that the rebels, backed by Russian arms and soldiers, had taken "advantage of the fact that our forces complied with the cease-fire."
Klimkin said Ukraine would demand that rebels return to the Minsk "separation line" at future meetings of a so-called "contact group" that includes separatist leaders from Donetsk and Luhansk.
Russia, for its part, on January 20 denied new accusations by Kyiv that Moscow sent troops into eastern Ukraine.
A Defense Ministry spokesman said that Kyiv's claim that some 700 Russian troops had crossed into eastern Ukraine was "absolute nonsense."
Also on January 20, Russian President Vladimir Putin said that the country must strengthen its armed forces to protect its sovereignty against the "challenge" posed by other countries that might threaten Moscow.
And the Russian Foreign Ministry the same day accused the European Union of maintaining an "unfriendly course" towards Moscow after Brussels said it would keep in place economic sanctions against Russia for its actions in Ukraine.
Meanwhile, Russian Prime Minister Dmitry Medvedev and Gazprom CEO Aleksei Miller said in a meeting that Kyiv still owes Russia some $2.44 billion for previous gas shipments and that it must settle its debt.
Miller also announced that, beginning on April 1, Ukraine would have to revert to paying a higher price for natural gas.
Ukraine paid some $378 per 1,000 cubic meters of gas in the last quarter of 2014 and is paying $365 per 1,000 cubic meters from January 1 until March 31.
Kyiv is expected to have to pay more than $100 extra per 1,000 cubic meters on April 1, putting a further strain on Ukraine's nearly bankrupt economy.
With reporting by Reuters and AFP