Nato Secretary-General Jens Stoltenberg has decried a "serious" military buildup of Russian forces, both within Ukraine and along its borders with Russia, and called on the Kremlin to pull back.
Stoltenberg said Nato had seen troops, equipment, tanks, artillery and air defence systems being moved in violation of a ceasefire agreement.
The Nato chief told reporters he had information on a buildup of Russian forces within Ukrainian territory.
"But we also see a military buildup on the Russian side of the border... This is a serious military buildup and we call on Russia to pull back its troops," he said, as quoted by Reuters news agency.
Russia has regularly denied providing arms or manpower to the pro-Russian separatists that have taken over parts of eastern Ukraine, including much of the cities of Donetsk and Luhansk.
Russia denied similar allegations that were made last week, after Nato's chief commander reported that military equipment from Russia had arrived in eastern Ukraine.
Meanwhile, Ukraine's Prime Minister Arseniy Yatsenyuk has called for talks with Russia on "neutral territory."
"We invite the Russian Federation to hold serious negotiations on a neutral territory. The US and EU are helping us with this," Yatsenyuk said, as quoted by Interfax Ukraine news agency.
For its part, the separatist leadership has called for new talks in the Belarussian capital Minsk, the same city where a ceasefire between the two sides was initially signed in September.