The UN Security Council has held an emergency session to discuss the crisis in Ukraine after NATO accused Russia of sending fresh columns of tanks, troops and military hardware into the former Soviet state.
The NATO 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.
US ambassador to the UN, Samantha Power, said Russia's actions in Ukraine were the "root of the problem".
"If our message sounds familiar it is for a good reason," she 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."
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."
The emergency meeting comes following statements from NATO's commander in Europe, US General Philip Breedlove, saying Russian military equipment was entering eastern Ukraine.
"Across the last two days we have seen the same thing that OSCE is reporting. We have seen columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops entering into Ukraine," he said during a visit to Sofia.
Russian denial
That was swiftly dismissed as "unfounded" by Russia's defense ministry. The Kremlin denies that it is involved in the fighting which has rocked east Ukraine since early April.
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 Defense 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.
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The Ukrainian government said it was preparing for an all-out war in the restive east of the country, as Nato accused Russia of funneling large amounts of military equipment, including tanks and troops, into the region.
Aljazeera