The US ambassador to the United Nations has accused Russia of "fuelling war" in Ukraine.
Speaking at a UN Security Council meeting in New York, Samantha Power said that NATO had confirmed sightings of Russian military equipment and troops entering Ukraine in the last 48 hours.
"If our message sounds familiar it is for a good reason," Power said at the meeting. "The situation has evolved, but the root of the problem remains the same: Russia's flagrant disregard for Ukraine's sovereignty and territorial integrity."
The meeting was the 26th time the Security Council had convened in an emergency session over Ukraine, again without taking action.
Russia's deputy representative, Alexander Pankin, warned that council meetings should not turn into a farce and accused his fellow council members of "yet another foray into propaganda with new flourishes".
The Security Council has met repeatedly on Ukraine since the crisis began early this year but little action can be taken as Russia, as a permanent member, has veto power.
For weeks, journalists, analysts, even the OSCE, have been seeing large convoys of military hardware on the move in rebel-held territory. Troops, Kamaz trucks, T-72B tanks, GRAD rocket launchers, artillery pieces. This is clearly not material looted from the Ukrainian army, as much of it is brand new, and some of them are modifications only used by the Russian military.
So if what Breedlove [NATO's commander in Europe] has seen isn't new, why is it news? Because he's the Commander of NATO, that's why. And NATO has generally pulled its punches when it comes to accusing Russia of what basically amounts to an invasion of sorts. Not any more. So, will Russian tanks be in Kiev by New Year? The military experts I've spoken to in Moscow say this is unlikely.
The wet autumn weather, low cloud, and where we are in the Russian troop conscription cycle all suggest the timing isn't right for a Big Push. But that doesn't mean we aren't about to see a significant upsurge in fighting. Indeed, this has already begun in both Donetsk and Luhansk regions. Both the rebels and Ukraine's army are gearing up for renewed hostilities. And the ceasefire agreed in Minsk in September is pretty much dead and buried.
NATO's accusation came amid growing fears of a return to all-out conflict in the region despite a two-month ceasefire which has stopped much frontline fighting but not shelling at strategic flashpoints.
General Philip Breedlove, NATO's commander in Europe, said they had seen "columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops entering into Ukraine".
"We do not have a good picture at this time of how many. We agree that there are multiple columns that we have seen," he said in Sofia, Bulgaria's capital.
That was swiftly dismissed as "unfounded" by Russia's defence ministry. The Kremlin denies that it is involved in the fighting which has rocked eastern Ukraine since early April.
Al Jazeera's James Bays, reporting from New York, said the UN representative had described the situation as being "on a knife's edge".
"The UN fears two things," he said. "One being a frozen conflict, the other is that we could return to the situation before the ceasefire agreement in September, and indeed, the UN representative has said the situation is not that different, with it becoming more and more violent by the day.
"It is felt that securing the border with Russia is key to securing the situation in Ukraine right now."
Al Jazeera's Rory Challands, reporting from Moscow, said it was not a good time for military action on either side.
"It is not good timing for a military offensive on either side, with winter fast approaching. This influx of military hardware is more intended to keep the status quo and stop the rebels being defeated.
"Fighting is likely to increase but it is unlikely we will see tanks in Kiev come Christmas time."
Also on Wednesday, Ukrainian Defence Minister Stepan Poltorak said Kiev was getting ready for a possible new round of fighting after seeing the "increased activity" by Russia and pro-Moscow rebels in the east.
"The main task I see is to prepare for combat operations. We are doing this, we are readying our reserves," Poltorak said at the start of a cabinet meeting.
"We observe their movements, we know where they are and we expect unpredictable actions from them."
He described the situation in the conflict zone as "complicated but stable".
The conflict in eastern Ukraine has claimed more than 4,000 lives since April, according to UN figures.
Source: Al Jazeera and agencies