Russian Foreign Minister Sergei Lavrov said on Friday that allegations Russia's military is fighting in eastern Ukraine are "conjecture".
"We're hearing various conjectures, not for the first time, but not once have any facts been presented to us," he said at a news conference.
Lavrov's comment came hours after Russian President Vladimir Putin called on pro-Russian rebels - who agreed - to open a humanitarian corridor to allow encircled Ukrainian troops to escape from conflict in the country's east.
Kiev has accused Russian troops of illegally entering eastern Ukraine and, backed by its U.S. and European allies, has said it will fight to defend its soil.
Russia stands accused of pushing troops and weapons into the former Soviet republic to shore up a separatist rebellion that a week ago appeared to be on its last legs. That development has sharply escalated the five-month conflict over eastern Ukraine.
In turn, NATO Secretary-General Anders Fogh Rasmussen said that Russian forces are engaged in direct military operations inside Ukraine in a blatant violation of Ukraine's sovereignty and territorial integrity.
"Despite Moscow's hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine," Rasmussen said.
"This is not an isolated action, but part of a dangerous pattern over many months to destabilize Ukraine as a sovereign nation," he added.
He said NATO would fully respect any decision by the Ukrainian parliament to abandon Ukraine's non-aligned status and set the country on a course to seek to become a member of US-led NATO.
For his part, Russian President Vladimir Putin said Kiev's operation in eastern Ukraine where government forces are fighting pro-Russian separatists around the cities of Donetsk and Luhansk is reminiscent of the World War Two Nazi siege of Leningrad.
"Small villages and large cities surrounded by the Ukrainian army which is directly hitting residential areas with the aim of destroying the infrastructure... It sadly reminds me the events of the Second World War, when German fascist... occupants surrounded our cities," Putin told a youth camp outside Moscow.
Putin noted that Kiev must sit down for negotiations with representatives of separatist eastern Ukrainian regions in order to achieve peace.
"It is necessary to force the Ukrainian authorities to substantively begin these talks -- not on technical issues ... the talks must be substantive," Putin added.
The Russian president pointed out that the Ukraine crisis was a tragedy and that Ukrainians and Russians are "practically one people".
"People who have their own views on history and the history of our country may argue with me, but it seems to me that the Russian and Ukrainian peoples are practically one people," he stated.
On another note, a leader of pro-Moscow rebel forces in eastern Ukraine, Alexander Zakharchenko, said on Friday he had agreed to open a humanitarian corridor for encircled Ukraine troops after Russian President Vladimir Putin's statement.
"We are ready to give a humanitarian corridor," Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, told Rossiya 24 TV, adding that troops would have to leave their heavy armored vehicles and ammunition.