DONETSK, Ukraine —Ukrainian fighters on Tuesday published a drone video that they claimed shows a Russian military base hidden near the front lines in war-torn eastern Ukraine.

The video, posted to YouTube by the Kiev-backed Dnipro-1 volunteer regiment, shows a large, fortified military encampment spread across a forested area two kilometers south of the village of Sontsevo, in the Donetsk region which is controlled by Russian-backed separatists.

Two flights were made over the area two weeks apart — on May 20 and June 4 — and the footage appears to show a buildup of forces at the site.

The video appears to show dozens of troops, several T-72 battle tanks (one with mine-sweeping equipment) and other towed artillery pieces as well as fuel tanks, transport trucks and engineering vehicles, including a front-end loader.

Several large tents and roads that appear either paved or graveled and lined with reflectors are also visible. Labyrinths of trenches and snaking vehicle track marks can be seen in surrounding fields.

Soldiers are seen standing near military trucks at a base in eastern Ukraine spotted by a Ukrainian drone.

The location of the base is important because it is strategically placed between the separatist stronghold of Donetsk, a city with a pre-war population of 1 million people, and Mariupol, the region's second-largest city with 500,000 people, which is still held by Ukraine.

Located on the Sea of Azov, Mariupol is strategic in that is has a working port with access to the Black Sea as well as steel and electrical plants that fuel much of the Donetsk region.

Positioned a mere 12 kilometers from the conflict line, a lateral offensive move from the newly discovered military base could be carried out quickly and without much warning to cut off a strategic north-south highway controlled by Kiev.

That would be an important step should the separatists decide to advance on Mariupol. And Donetsk separatist leader Alexander Zakharchenko has been very open about his desire to seize more territory in the region.

Battle tanks parked at the military base.

But an offensive of that kind would require significant Russian military support in the way of soldiers and weaponry. And to hold any new ground would require even more. The number of troops and armor seen in the drone video is unlikely to be enough.

Still, with Ukraine's border with Russian wide open, there would be no way to stop armor and troops moving into the territory from Russia.

The Organization for Security and Co-operation in Europe (OSCE) has reported seeing significant quantities of both moving inside the Donetsk region in the direction of the front line and toward the vicinity of this base in recent weeks.

And just last week the top NATO commander, Air Force Gen. Philip Breedlove, warned of a "constant flow" of troops and materiel across the border.

"What smart people are saying is that Mr. Putin is very clear that he does not want Kiev leaning to the West and he will use the appropriate force necessary to keep Kiev from leaning to the West," said Breedlove.

Russian President Vladimir Putin has repeatedly and vehemently denied backing the separatists in eastern Ukraine. But Western governments say otherwise, and an ever-growing body of evidence that shows Russian soldiers and military equipment to have been sent into the region and used to fight against Kiev's government forces backs up their words.

That fighting continues to rage on across the front line, which cuts through the Donetsk and Luhansk regions, despite a ceasefire agreed in February. More than 6,400 people have been killed in the conflict since April 2014.