On Thursday, senior Ukrainian officials, including President Petro Poroshenko, accused Russia of allowing tanks and heavy artillery to cross into Ukraine in what could be a significant escalation of the conflict.
The U.S. and the U.K. on Friday backed up Ukraine's claim.
U.K. Foreign Secretary William Hague said the entry of three Russian tanks into Ukraine was "completely unacceptable" and warned of further Western sanctions "if Moscow continues to provoke instability."
Ukraine's Interior Minister Arsen Avakov said Thursday that a "column" of armored vehicles had crossed from Russia through border-control points controlled by separatists near the village of Dyakove in eastern Ukraine.
He said three tanks went to the town of Snizhne, about 25 miles from Dyakove, one vehicle stayed at the border and two headed toward Horlivka.
The newly released images, which come from open sources including commercial satellite contractor DigitalGlobe Inc. and from videos posted on YouTube, were provided by a NATO military official.
Most of the images are grainy and it is difficult to independently verify the details provided by the official.
The NATO official said two images show a Russian military staging area near Rostov-na-Donu in southwest Russia, some 56 miles by road from the Ukrainian border area where the tanks allegedly crossed.
One image, dated May 30, shows a Russian unit on site, with dozens of administrative and armored vehicles but no battle tanks, the official said.
A second image, dated June 6, showed most of the vehicles gone— possibly as part of the Russian pullback of forces from the Ukrainian border, the NATO official said.
However the image shows what appear to be eight new vehicles on site that the official said are battle tanks.
A third image, dated June 11, shows 10 battle tanks on site, the NATO official said, three of them parked and four in a training area.
A magnified section shows what the official said are three other battle tanks sitting on heavy-equipment-transporter trucks—vehicles used to transport battle tanks.
The image suggests the "imminent movement by road" of the three tanks just hours before Ukraine claimed vehicles had crossed the border, the official said.
The official also provided several screenshots from YouTube videos.
One showed two unmarked T-64 tanks along with a vehicle flying a Russian flag.
The image came from the Ukrainian town of Makiyivka, on the outskirts of the separatist stronghold of Donetsk, the NATO official said, but the official didn't provide any other information on the source of the YouTube videos.
Another image showed what the official said was an unmarked T-64 tank in Snizhne.
Unmarked tanks with all identifying insignia stripped off were widely used in Russia's takeover of Crimea.
A final image contrasted the picture of the T-64 in Snizhne with the design of T-64 tanks used by the Ukrainian army.
The Ukrainian tank has a camouflaged paint pattern which contrasts with the T-64 seen in Snizhne.
The armor around the tank turret also appears different.
The official said NATO also has additional confidential images suggesting the tanks came from Russia.
"These images raise significant questions concerning Russia's role in facilitating instability in eastern Ukraine and its involvement in the movement of military equipment from Russian territory into Ukraine," said the NATO official.
The official said that if the reports of the tank incursion are confirmed, it would "mark a grave escalation of the crisis in eastern Ukraine."
Russia has consistently denied it is arming separatist groups in Ukraine and has demanded that Kiev scrap its military operation against the separatists.
Russian President Vladimir Putin spoke to his Ukrainian counterpart on Thursday.
The NATO images appear consistent with evidence cited by a State Department spokeswoman, Marie Harf, on Friday.
Ms. Harf cited video evidence of a convoy of T-64 tanks in eastern Ukraine coming from a site in southwestern Russia, as well as BM-21 rocket launchers.
"Russia will claim these tanks were taken from Ukrainian forces but no Ukrainian tank units have been operating in that area," she said.
"We are confident that these tanks came from Russia."
The U.S., Germany and France on Saturday continued to press Moscow to stem the flow of weapons across the border.
German Chancellor Angela Merkel and French President François Hollande urged Mr. Putin in a joint phone call to "control its borders with Ukraine," a German spokesman said.
U.S. Secretary of State John Kerry expressed "strong concern" about the flow in a call with his Russian counterpart, a U.S. official said.
Source: The Wall Street Journal