Nato has seen Russian military equipment and Russian combat troops entering Ukraine this week, its top commander Gen Philip Breedlove says.
Nato chief Jens Stoltenberg later urged Russia to pull back its forces.
Russia's defence ministry denied sending troops to eastern Ukraine to help pro-Russian separatists there.
The European OSCE monitoring mission has warned of "a real risk" of further escalation in a conflict that has killed more than 4,000 people.
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.
Analysis: Jonathan Marcus, BBC defence and diplomatic correspondent
Events in Ukraine seem to be turning full circle.
Back in August, Nato was warning about the deployment of Russian artillery batteries inside Ukraine, the supply of Russian military equipment to the rebel forces and the build-up of further Russian combat units at the Ukrainian frontier.
Since then many of these units have been withdrawn.
But now with tensions renewed, Nato's Supreme Commander in Europe General Philip Breedlove has confirmed that over the past two days, Nato has seen columns of Russian armour, artillery and crucially - combat troops - entering Ukraine.
The question now is whether this is just a re-run of events in the summer or does a more significant clash beckon, perhaps one where the Kremlin may decide - in its terms - to teach the Ukrainians a military lesson.
General Breedlove also confirmed that Nato believes Russia is deploying nuclear-capable weapons to Crimea - a reference to reports that Russia is deploying short-range Iskander ballistic missiles there that could potentially be equipped with nuclear warheads.
The Organization for Security and Co-operation in Europe (OSCE) has reported seeing unmarked convoys in the region in recent days.
This was confirmed by US Gen Breedlove on Wednesday, who said "Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops" had been sighted.
Gen Breedlove, talking to reporters on a visit to Bulgaria, said the alleged Russian troop deployment may be intended to reinforce "pockets" under separatist control in eastern Ukraine.
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 were seen. A Nato official confirmed to the BBC that Nato had "assessed" that the equipment and troops were Russian in origin.
In a separate statement, Nato Secretary General Jens Stoltenberg said any significant military build-up would be "a severe threat to the ceasefire", which was agreed in Minsk on 5 September.
However, Russian defence official Maj-Gen Igor Konashenkov said "there was and is no evidence" to support Gen Breedlove's claims.
Russia has consistently denied sending troops and equipment to support the rebels fighting in eastern Ukraine. However, the rebels have admitted being helped by "volunteers" from Russia.
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.
"The main task I see is to prepare for combat operations. We are doing this, we are readying our reserves," Stepan Poltorak told a government meeting.
The UN's assistant secretary general, Jens Anders Toyberg-Frandzen, later warned of the possibility "of a return to full-scale fighting" in Ukraine, speaking at an emergency session of the UN Security Council on Wednesday.
More than 4,000 people have died since government forces moved in April to put down an armed insurrection by the rebels in the two regions, which border Russia.
Hundreds of people have been killed since a fragile truce was agreed three months ago.
German Chancellor Angela Merkel said on Tuesday her government was "not satisfied" with the progress in implementing the Minsk agreement, but added that there were no plans at present for further economic sanctions against Russia over its involvement in Ukraine.
"Further economic sanctions are not planned at the moment, we are focusing on the winter and the humanitarian situation there and how to get a real ceasefire," she said.