The US has accused Russia of undermining a ceasefire in Ukraine after reports of Russian
troops and military hardware entering the country.
The US envoy to the UN, Samantha Power, also accused Russia of pursuing war in Ukraine while talking peace.
Russia's deputy ambassador to the UN, Alexander Pankin, dismissed the allegations as propaganda.
More than 4,000 people have died in the conflict between Ukraine's military and pro-Russian rebels in the east.
Nearly a million people have been driven from their homes since the fighting started in April.
The violence has continued despite a ceasefire deal struck in the Belarusian capital, Minsk, in September, with both sides accusing each other of violations.
Relations between Russia and the West are at their lowest ebb since the Cold War as a result of the conflict.
As Nada Tawfik reports, the Kremlin has denied Nato's claims
The US called a meeting of the UN Security Council on Wednesday after Nato claimed Russia was sending troops, artillery and air defence systems across the border into Ukraine.
"The pattern is clear," Ms Power told the meeting. "Where Russia has made commitments it has failed to meet them."
"Russia has negotiated a peace plan and then systematically undermined it at every step. It talks of peace but it keeps fuelling war."
However, Mr Pankin said the Nato claims "do not reflect the situation on the ground". He accused security council members of a "foray into propaganda".
Meanwhile, Jens Toyber-Frandzen, the UN assistant secretary-general for political affairs, also warned of "a return to full-scale fighting".
A column of unidentified tanks was seen on a road near the rebel-held town of Shakhtarsk on Monday He said an alternative scenario - a simmering conflict "with
sporadic low-level battles" - would also be "a catastrophe for Ukraine".
He also expressed concern at a third prospect - "a frozen or protracted conflict that would entrench the status quo in south-eastern Ukraine for years or decades to come".
Nato's top commander, US Gen Philip Breedlove, told reporters on Wednesday that the Russian troop deployment in Ukraine may be intended to reinforce "pockets" under separatist control.
This could, he said, help them to form "a more contiguous, more whole and capable pocket of land in order to then hold on to it long term".
He did not specify how many troops, vehicles or weapons had been seen. A Nato official confirmed to the BBC that Nato had "assessed" that the equipment and troops were Russian in origin.
US Gen Breedlove, Nato: "What worries me most is that... the border between Ukraine and Russia is completely porous"
However, Russian defence official Maj-Gen Igor Konashenkov said "there was and is no evidence" to support Gen Breedlove's claims.
The current crisis in Ukraine can be traced back to the overthrow of a pro-Russian president by street protests in February.
Russia responded with the annexation of the Ukrainian territory of Crimea. Pro-Russian rebels in eastern Ukraine also began a separatist uprising against the government.
Russia has consistently denied sending troops and equipment to support the rebels. However, the rebels have admitted being helped by "volunteers" from Russia.
The main city in Ukraine's east, Donetsk, has seen its heaviest shelling in weeks
Separately, Russian Defence Minister Sergei Shoigu said that the country's long-range aircraft would go on patrol flights over the Arctic Ocean to the Caribbean and the Gulf of Mexico.
He said that the current situation required Russia to restart the flights, which were cut at the end of the Cold War.
Meanwhile, Ukraine's defence minister has said that government forces are redeploying in preparation for a possible new offensive by pro-Russian separatist rebels.
Heavy artillery fire rocked the east Ukrainian city of Donetsk, the industrial hub held by pro-Russian separatist rebels, on Wednesday.
There were also reports of fighting near the rebel-held city of Luhansk, with one Ukrainian soldier killed and another injured, according to Ukrainian security forces.
