The Organization for Security and Cooperation (OSCE) says it is "very concerned" after its monitors witnessed columns of military equipment moving through territory in east Ukraine controlled by pro-Russia separatists.
In a statement issued on November 8, the OSCE said its monitors had observed "convoys of heavy weapons and tanks" in the rebel-held city of Donetsk and nearby Makiivka.
It comes hours after the AP news agency said its reporters had seen more than 80 unmarked military vehicles near Donetsk on November 8.
The OSCE report also came a day after Ukraine's military accused Russia of sending a column of 32 tanks and truckloads of troops into the country's east to support pro-Russian separatists fighting government forces.
Russia has denied backing the rebels with fighters or arms.
"More than 40 trucks and tankers" were seen driving on a highway on the eastern outskirts of Makiivka, the OSCE monitors said.
The OSCE said 19 of these vehicles were large trucks without markings or number plates, and carrying personnel in dark green uniforms as well as towing howitzers.
Separately, the OSCE monitors also said they had seen "a convoy of nine tanks -- four T72 and five T64 -- moving west, also unmarked," just southwest of Donetsk.
The Swiss foreign minister and OSCE chairperson-in-office, Didier Burkhalter, said he was "very concerned about a resurgence of violence in the eastern regions of Ukraine."
"He urged all sides to act responsibly and to do all in their power to further consolidate the cease-fire," the OSCE said in a statement.
The OSCE monitors have been tasked with monitoring the shaky cease-fire signed by Kyiv and the pro-Russia separatists in Minsk in September.
AP said its reporters had taken photos and video of three columns of armed vehicles and other military hardware: one near the city of Donetsk and two near Snizhne.
Most of the vehicles were transport trucks but there was at least one armored personnel carrier.
AP said several of the trucks were carrying troops.
Earlier on November 8, U.S. Secretary of State John Kerry said that the United States and Russia had agreed to exchange information about the situation on the Russia-Ukraine border.
Kerry met with Russian Foreign Minister Sergei Lavrov in Beijing ahead of an Asia-Pacific Economic Cooperation summit.
Lavrov deflected a question about whether Moscow was sending tanks and troops to help the separatists.