Reports that the Russians are moving military hardware some 250 kilometers from the border have raised eyebrows in Washington. And Ukrainian forces have deployed combat drones along the battle lines that separate them from pro-Russian separatists in the Donbas region.

The front lines of the conflict -- a standoff stretching hundreds of miles from northern Ukraine to the Sea of Azov -- have barely moved in five years.

On Monday, US Defense Department spokesman Admiral John Kirby said the Pentagon was "aware of public reports of unusual Russian military activity near Ukraine."

Satellite imagery has shown Russian hardware -- including self-propelled guns, battle tanks and infantry fighting vehicles -- on the move at a training ground roughly 186 miles (300km) from the border.

But Ukraine's Defense Ministry said Monday it had recorded no "additional transfer of Russian units, weapons and military equipment to the state border of Ukraine."

On Tuesday, the Defense Ministry said an estimated 90,000 Russian troops were located "near the border and in the temporarily occupied territories" as well as in the Black Sea.

Ukraine's Defense Ministry added that Russia had established a practice of "transferring and accumulating military units for the purpose of maintaining tension in the region and political pressure on neighboring countries."

Kirby said the US was watching closely: "I can't speak for Russian intentions, but we are certainly monitoring the region closely, as we always do. Any escalatory or aggressive actions would be of great concern to the United Sates."

On Tuesday, Kremlin spokesman Dmitry Peskov told reporters: "The movement of our military equipment or army units across the territory of the Russian Federation is exclusively our business.

"Russia has never threatened anyone, is not threatening, and does not pose a danger to anyone," he insisted.

Observers say Russia's actions are worth keeping a close eye on.

"At the moment it is a developing situation. It's not 'nothing happening' and it doesn't mean that there will be an offensive op tomorrow," says Michael Kofman, a fellow at the Wilson Center who researches Russia's military.

Closer view of armored units and support equipment in Yelnya, Russia.

'Provocation' claims

We've been here before -- several times -- since the separatists, with Russian backing, entered eastern Ukraine in 2014.

Heightened tensions have, in the past, come to nought. Russian units gathered near the border last spring -- setting off alarm bells in western capitals -- but eventually returned to base.

But hopes that the frozen conflict might be defused through negotiations sponsored by European governments and the US are moribund.

Russia has responded swiftly to Ukraine's use of Turkish-made combat drones for the first time in the conflict. One of those drones struck a separatist position last week.

"We observe attempts to carry out provocations, elicit some reaction from the militia and drag Russia into some kind of combat action," Foreign Minister Sergei Lavrov told Russian state television Monday.

Popular Russian TV host Vladimir Soloviev went further, saying Ukraine was provoking the separatist-held "Republics" into taking "retaliatory measures, which means a major war. Under these circumstances, Moscow will be confronted with a serious choice."

Russian rhetoric towards Ukraine has hardened in recent months.

Both President Vladimir Putin and his predecessor, Dmitri Medvedev, have penned essays describing Ukraine as a vassal of the West -- even going so far as to suggest it is not a real country.

In a long article in July, Putin said "the formation of an ethnically pure Ukrainian state, aggressive towards Russia, is comparable in its consequences to the use of weapons of mass destruction against us."

"True sovereignty of Ukraine is possible only in partnership with Russia," he wrote.

Self-propelled artillery is pictured in this image supplied by Maxar.

Combined arms units are pictured in Yelnya, western Russia on November 1 in an image supplied by Maxar.

Pivot from east to west

Moscow's strategy is aimed at deterring Ukraine's flirtation with closer ties to NATO and the European Union, a pivot from east to west that would stoke historic Russian fears of encirclement.

Those fears deepened when the former Soviet Baltic states joined NATO, along with several ex-members of the Warsaw Pact, such as Romania and Poland.

Just last month, US Defense Secretary Lloyd Austin reiterated US support for Ukraine's "reorientation," pledging "continued US support for Ukraine's sovereignty, territorial integrity and Euro-Atlantic aspirations."

Ukraine joining NATO would be an "extremely dangerous" move that would trigger retaliatory measures from Russia, Russian Deputy Foreign Minister Andrey Rudenko retorted.

Speaking to journalists on Tuesday, Peskov condemned what he said were "aggressive expansionist tendencies, especially on the part of NATO and other countries," adding that "Russia has always taken measures to ensure its security and will continue to do so."

The US, which already supplies Javelin anti-tank missiles to Ukraine, also promised to support Ukrainian forces through security assistance, including efforts to enhance the country's maritime capacity. US warships regularly patrol the Black Sea -- much to Russia's ire.

Russia's latest weapon in the conflict is not camo-painted. Instead, it comes through a pipeline. The Nord Stream 2 (NS2) gas pipeline connecting Russia to Germany is near completion, and that worries Ukraine -- which has been a transit route for Russian gas through its extensive network of pipelines.

"Putin is telling everyone to their face: 'You allow NS2 to become operational or you won't get any more gas,'" according to Yuriy Vitrenko, chief executive of Ukraine's Naftogaz.

Vitrenko told the Financial Times that "[If] there will be no physical transit going through Ukraine, it increases the chances of a full-scale war between Russia and Ukraine."

The White House said at the weekend that US President Joe Biden had spoken with German Chancellor Angela Merkel about efforts "to ensure that Russia cannot manipulate natural gas flows for harmful political purposes."

As yet, there are no signs that all the adversarial language is translating into a higher level of hostilities along what's known as "the line of contact," but Russian calculations are never easy to gauge.