NATO is calling on Russia to cease its illegal military actions in Ukraine and stop its support to armed separatists.
The comments follow an extraordinary meeting of the NATO-Ukraine Commission.
NATO says that despite Moscow denials, "it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine. This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation."
It says Russian forces "are engaged in direct military operations inside Ukraine", and that Russia continues to supply the separatists with tanks, armoured vehicles, artillery and rocket launchers.
NATO says all this "is a blatant violation of Ukraine's sovereignty and territorial integrity. It defies all diplomatic efforts for a peaceful solution."
The NATO Secretary-General Anders Fogh Rasmussen has said "Today, we expressed strong solidarity with Ukraine".
Mr Rasmussen has called on Russia to stop what it is doing.
It comes after the Russian Foreign Minister Sergei Lavrov dismissed claims that its troops are fighting in eastern Ukraine as "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.
Mr Lavrov was speaking after the US accused Russia of lying about its involvement in Ukraine and warned of tougher economic sanctions.
Speaking at the White House, US President Barack Obama said Russia has been supporting pro-Moscow separatists in eastern Ukraine "for months".
He ruled out any direct US military intervention but said Russia would incur "more costs and consequences".
"I consider the actions that we've seen in the last week a continuation of what's been taking place for months now," Mr Obama said.
"The separatists are backed, trained, armed, financed by Russia. Russia determined that it had to be a little more overt in what it had already been doing, but it's not really a shift."
Kiev said on Friday Russia's call for separatists to open a 'humanitarian corridor' to allow encircled Ukrainian troops to withdraw was further proof they were "led and controlled directly from the Kremlin".
The comments come as Foreign Affairs Minister Charlie Flanagan is attending a two day meeting of his EU counterparts in Italy.
The meeting will discuss a range of issues including Gaza and the Middle East Peace Process, Iraq, Syria and the situation in Ukraine.
Mr Flanagan says he will be emphasising Irish strong support for Ukraine - and the need for a political solution.
Speaking to Newstalk Breakfast earlier, Professor Paul Rogers - professor of peace studies at Bradford University - outlined the current situation in the region.
The US President, meanwhile, told reporters Russia cannot pretend it has nothing to do with the crisis in the region.
A spokesman for Ukraine's National Security Council, Colonel Andriy Lysenko, said two columns of tanks had fired at a border post, before rolling into Ukraine.
His comments backed up Ukrainian President Petro Poroshenko's earlier report that Russian forces had "entered" the country.
The tanks are reported to have crossed into a southern area of the Donetsk region, where the separatists have made significant gains in recent days.
Originally posted 6:05am
