A key figure in covert Russian efforts to destabilize eastern Ukraine is working for Russia’s GRU military intelligence service, according to U.S. officials.

Igor Vsevolodovich Girkin, who is currently using the nom de guerre Igor Strelkov, has identified himself in press reports as "defense minister" of the self-proclaimed, pro-Russian "Donetsk People’s Republic. This week he called for Russia to send military aid to rebels in eastern Ukraine.

U.S. officials said Strelkov is considered a leading pro-Russian rebel who is undermining stability as part of a major Russian covert action program in Ukraine.

Officials said Girkin is a GRU colonel who was first identified by the Ukrainian SBU intelligence service. Girkin traveled to the Crimea in February and traveled to Donetsk in April. While in the Crimea was a military adviser to Sergei Aksenov, the current head of Russian-occupied Crimea.

He is also a close aide to the so-called Donetsk prime minister, Alexander Borodai, who is also a Russian. Borodai also worked with Aksenov in Crimea, an indication the Russians may be planning a similar annexation in the Donetsk region.

Girkin and Borodai were employed by a Moscow investment firm called Marshall Capital. That company is owned by Konstantin Valeryevich Malofeyev, who has been linked to funding of pro-Russian separatist activities in Crimea and eastern Ukraine.

On Friday, the Treasury Department imposed financial sanctions on Girkin and six other Ukrainian separatists for their role in seeking to destabilize Ukraine.

"The United States will continue to take action to hold accountable those persons engaged in efforts to destabilize Crimea and eastern Ukraine," said David S. Cohen, Treasury undersecretary for terrorism and financial intelligence. "These individuals have all contributed to attempts to illegally undermine the legitimate government in Kyiv, notably by falsely proclaiming leadership positions and fomenting violent unrest."

Girkin led the abduction of military observers in Ukraine, and attacked military brigade and stole a large cache of weapons, Treasury said in a statement.

Disclosure of GRU activities in Ukraine comes as the chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey, criticized Moscow’s covert subversion.

"The tactic that Russia is using is one I would describe as proximate coercion, subversion, and misinformation," Dempsey told a Senate defense appropriations subcommittee on Wednesday.

The strategy is to mass forces on the border and threaten the use of force and employ subversion through surrogates and proxies, along with misinformation to arouse ethnic populations, he said.

"I doubt that there'll actually be a full-blown invasion," he said. "But we've got to be alert to the other tools that [Putin] may use to actually undermine stability, notably in the Baltics and in some of our Eastern European allies."

Russia has "lit a fire" in Ukraine that is burning out of control and the country is facing a "very difficult path," Dempsey said.

In London, NATO Secretary General Anders Fogh Rasmussen said Thursday that Russian troops were again building up along Ukraine's eastern border, and Russian forces were seen conducting troop maneuvers nearby.

U.S. officials familiar with intelligence reports said Russian covert action programs have been underway in Donetsk and Luhansk, as well as other areas.

The rebels downed a Ukrainian military helicopter on May 29, killing a key Ukrainian general and demonstrating they have advanced anti-aircraft weapons.

The rebels are known to have portable anti-aircraft missiles, anti-aircraft artillery, and armored troop carriers.

The missile that downed the helicopter could not be confirmed as having been supplied by Russia, but other arms found after battles with rebels have revealed they were recently shipped from the Russian military.

Intelligence reports also indicate that Russia is sending Russian troops disguised as Ukrainian to support pro-Russian rebels in the region. Rebels have described the Russians as "volunteers," including Cossacks from southern Russia. Similar tactics were used prior to the takeover of Crimea in March.

Another indicator of covert Russian forces is the discovery of a unit called the Vostok, or East, Battalion, a well armed group of ethnic Chechen special operations commandos who are fighting with the rebels.

An earlier Vostok force was set up by the Russian military using ethnic Chechen special operations commandos who specialize in urban warfare and who were led by Russian officers. The unit was dispatched to earlier conflicts in Chechnya and in the battle with Georgia in 2008.

Officials identified the border town of Antratsyt, near Luhansk, as a key base for the infiltration of both Russian fighters and weapons.

The newly elected government in Kiev this week offered to halt its military operations against eastern pro-Russian separatists. Newly-elected President Petro Poroshenko offered amnesty to rebels that surrender. The amnesty was limited to rebels that were not involved in capital crimes. Poroshenko also offered to allow Russian fighters to return to Russia.

State Department spokeswoman Jen Psaki told reporters Wednesday that the plan will only work if the Russian government supports it, something that has not happened so far.

"Russia of course must support the peace plan instead of continuing to support the separatists on the ground," she said.

On Tuesday, Psaki said there were reports of renewed Russian troop movements close to the Ukrainian border.

"Our concern has been focused more, naturally, on the actions of the Russian separatists and the movement of military tanks and other equipment across the border," she said.

Other reports indicated Moscow is covertly delivering arms, including tanks and rocket launchers, to rebels in eastern Ukraine, in addition to the intelligence operatives.

Regarding Girkin, the GRU officer was quoted in a Russian newspaper on Monday as calling for Moscow to send military aid to two so-called "people’s republics," in Donetsk and Luhansk. The eastern Ukrainian cities are strongholds for anti-government rebels.

"I am calling on Russia as the commander of the [Donetsk People’s Republic] militia and as a patriot of Russia and the Russian people. It can be considered as an appeal on behalf of the Donetsk militia," he told the Russian newspaper Komsomolskaya Pravda.

"If Russia does not take prompt and decisive action, they [the Ukrainian Army] will achieve their goal," he told another pro-Russian news outlet. "Several thousand militiamen near the Russian border cannot stand long against powerful enemy artillery and aviation."

Girkin said his militia lack funds and supplies. "We can ‘bite' the enemy, attack them from the flank, we can operate through sabotage and reconnaissance forces, but we cannot change the situation only with heroism," he told the pro-Russian LifeNews outlet.

The GRU, acronym for the Main Intelligence Directorate of the Russian General Staff, is playing the key role in Moscow’s efforts to destabilize Ukraine following the military annexation of Crimea earlier this year.

So far, several GRU officers have been exposed by Ukrainian authorities, including GRU Lt. Col. Kirill Koliuchkin, a naval attaché expelled by Kiev for spying activities.

The European Union in April imposed sanctions on more than a dozen Russian officials, including GRU Director Lt. Gen. Igor Sergun. The sanctions are largely symbolic but including Sergun on the list highlights the key role being played by GRU spies in Ukraine.

In addition to GRU operatives in eastern Ukraine, Russia’s Spetsnaz special operations commandos also are engaged in subversion.

Mark Galeotti, a New York University professor who writes the blog "In Moscow’s Shadows," stated in a recent post that the GRU has been given new life by Russian covert action in Ukraine.

The service, under Russian President Vladimir Putin, a former officer in the rival KGB, has faced large-scale cutbacks in personnel from 2009 to 2011.

Over 1,000 GRU officers and about 80 GRU generals were dismissed, retired, or transferred to other units, Galeotti stated. The cutbacks were the result of bureaucratic infighting with the civilian foreign spy agency SVR, acronym for Foreign Intelligence Service, and the domestic political police and spy agency known as the FSB, for Federal Security Service.

Galeotti said the GRU is a key element of Russian asymmetric military-political warfare tools. "They are less valuable as straightforward war-fighters and much more so as covert operators and the facilitators of other deniable operations," he said.

"Not only may the Ukraine conflict help stop–or at least bring a temporary ceasefire to–internecine struggles within the Russian security apparatus, it may well prove the savior of the GRU in its current configuration."