NATO says it has observed columns of Russian military equipment entering conflict-wracked eastern Ukraine in the past two days.
Speaking in Sofia on November 12, the alliance's top commander, U.S. General Philip Breedlove, said the columns included Russian tanks, artillery, air-defense systems, and combat troops.
"We do not have a good picture at this time of how many. We agree that there are multiple columns that we have seen," Breedlove said.
Breedlove made the comments after a report from the Organization for Security and Cooperation in Europe (OSCE) said its monitors had seen a convoy of unmarked military trucks -- some towing howitzer artillery pieces and multilaunch rocket systems -- travelling into the rebel stronghold of Donetsk on November 11.
On November 12, Michael Bociurkiw, spokesman for the OSCE Special Monitoring Mission to Ukraine, told RFE/RL's Ukrainian Service in Kyiv that monitors near Donetsk had observed 126 unmarked military vehicles since November 8.
"By unmarked, I mean no license plates. We also mean that the men inside these vehicles have no insignia on their uniforms. But they're towing very heavy equipment, and we're talking about multiple rocket launchers, we're talking about 126-milimeter howitzer guns. This is not what one expects after the signing of the Minsk accords," Bociurkiw said.
The Russian Defense Ministry later denied that Russian armed forces were present in Ukraine.
Interfax news agency quoted ministry spokesman Major General Igor Konashenkov as saying that "there was and is no evidence" behind such statements.
The sightings have fueled fears of an escalation in the conflict in eastern Ukraine, where fighting persists despite a September 5 cease-fire and more than 4,000 people have been killed since April.
They have added to what Kyiv and NATO say is overwhelming evidence of direct Russian military support for pro-Russian separatists who control large parts of Ukraine's Donetsk and Luhansk regions.
Russia denies involvement.
In related news, the OSCE says pro-Russian separatists have shot at some of its unarmed drones in eastern Ukraine.
OSCE Secretary-General Lamberto Zannier said on November 12 in Brussels that video footage recorded by the drones showed "nondescript militias wearing no identification [fighting] on the side of the rebels" firing at the unmanned aerial vehicles with antiaircraft weapons.
Zannier said that the drones were also subjected to "high-end military-grade jamming," but added that he did not know where the jamming originated.
The OSCE on October 24 began drone flights to record truce violations and monitor the general security situation in eastern Ukraine, where more than 4,000 people have been killed since April in a conflict between government forces and the rebels.
The conflict, which erupted after Russia annexed the Black Sea peninsula of Crimea in March following the ouster of a Russian-backed president and the advent of a pro-Western government in Kyiv, has brought ties between Moscow and the West to post-Cold War lows.
Fears Of Separatist Offensive
In Kyiv, Ukrainian Defense Minister Stepan Poltorak said government forces are redeploying in preparation for a possible offensive by pro-Russian separatists.
"We are repositioning our armed forces to respond to the actions of the fighters," Poltorak told a government meeting. "I see my main task is to prepare for military action."
A military spokesman said on November 11 the rebels were beefing up their forces, including around the Donetsk region port city of Mariupol, which is controlled by government forces and is seen by the separatists as a key prize.
Also on November 12, reports said heavy shelling rocked Donetsk.
An AFP correspondent said mortar shells were being fired from an area near the city center toward government-held positions around Donetsk's international airport.
Shelling occurs almost daily around the airport despite the September 5 truce, which Breedlove said on November 11 was a cease-fire "in name only."
With reporting by AFP and Reuters