Russia is sending additional military forces toward the border with eastern Ukraine, including units equipped with ballistic missiles, as part of Moscow’s ongoing destabilization effort in support of pro-Russian rebels.

U.S. officials with access to intelligence reports said one Russian military unit equipped with short-range ballistic missiles was detected this week near eastern Ukraine, where Russia has launched a destabilization program following its military annexation of Ukraine’s Crimea in March.

The military movements coincided with the an unusual number of flights last week by Russian strategic nuclear bombers and aircraft along Europe’s northern coasts in a what NATO’s military commander called strategic "messaging" toward the West.

"My opinion is that they’re messaging us," Gen. Phillip Breedlove, the commander, told reporters at the Pentagon this week. "They’re messaging us that they are a great power and that they have the ability to exert these kinds of influences in our thinking."

The bomber flights included three days of paired Tu-95 bomber flights that were to have circumnavigated Europe from the north but instead were halted near Portugal.

U.S. officials said Russia deployed several Il-78 refueling tankers in Egypt that were to resupply the bombers during flights over the Mediterranean, but those flights were scrapped for unknown reasons.

NATO Secretary General Jens Stoltenberg expressed concerns about Russian military moves in Ukraine during remarks to reporters Tuesday in Brussels.

"Recently we are also seeing Russian troops moving closer to the border with Ukraine, and Russia continues to support the separatists by training them, by providing equipment, and supporting them also by having special forces, Russian special forces, inside the eastern parts of Ukraine," Stoltenberg said.

Other officials said both intelligence and social media reports in recent days revealed an increase in Russian deployments.

The missile systems being deployed were described as conventionally armed, short-range ballistic missiles, multiple launch rocket systems, and BM-21 Grad multiple rocker launchers.

Additionally, Russian military forces are moving towed artillery pieces closer to the border.

One official said the display of military power is part of Moscow’s effort to reinforce "separatists" seeking to carve out a pro-Russian enclave in Eastern Ukraine.

The Russian "Spetsnaz" or special forces commandos are already inside the country, but the ground forces as of Wednesday appeared to be staging at the border.

Russian military forces in Ukraine number around 300 commandos. "These are not fighting formations. These are formations and specialists that are in there doing training and equipping of the separatist forces," Breedlove said.

The buildup is either part of a plan for military escalation, or a coordinated pressure tactic by Moscow to force Ukraine to make concessions to the rebels, officials said.

Rebel groups in the region have made repeated threats to take control of the key southeastern Ukrainian port of Mariupol and other territory unless the Ukrainian government agrees to make changes in the current separation line.

"The build up may just be a pressure tactic to force such concessions, or it may presage further escalation," one official said.

Rebels in eastern Ukraine recently held elections that Ukraine and NATO dismissed as illegal. New charges were raised in Kiev Wednesday about violations of a peace agreement reached in Belarus in September.

Breedlove said Monday there was no "huge change" in Russian deployments. Currently about seven battalion task groups are stationed near the border with Ukraine.

"Some of those formations have moved closer to the border," he said. "We believe that was probably to bring some pressure on and make sure that the elections went according to the separatist plans; we'll look now to see if they pull back from the border into their previous border locations."

"We have now realistically entered the phase of a ‘frozen conflict,’" Yury Yakimenko, a political analyst at Ukraine's Razumkov political research center told Reuters. The term frozen conflict has been applied to other former Soviet Republics where separatists are being backed by Russian forces.

Russia’s aggression against Ukraine is part of a program by Russian President Vladimir Putin to gain control or hegemony over former Soviet bloc states described as the "near abroad."

Putin is seeking to restore Russian power with territorial seizures, along with a large-scale nuclear and conventional forces buildup.

Pentagon spokeswoman Eileen M. Lainez said Russian forces and equipment remain on Ukraine’s border and on Ukrainian territory in violation of international law. "We again call on Russian authorities and the separatists they back to abide by their commitments under the Sept. 5 ceasefire agreement and the Sept. 19 implementing agreement," she said.

Breedlove said the Russians in the past have conducted small-scale bomber flights.

"And what you saw this past week was a larger, more complex formation of aircraft carrying out a little deeper and, I would say, a little bit more provocative flight path," he said. "And so it is a concern."

The flights are destabilizing and "problematic," Breedlove said.

Stoltenberg, the NATO secretary general, also voiced concerns about the Russian bomber flights.

"When it comes to the increased Russian military activity, both in the air but also along the borders of Ukraine, I think that what we see is, especially when it comes to increased air activity of Russian planes, is that they are showing strength, and what we are doing is what we are supposed to do: we are intercepting the Russian planes, whether it is in the Atlantic Sea or the Baltic Sea or in the Black Sea," he said.

Breedlove said he has discussed with U.S. military chiefs the idea of moving additional troops and supplies closer to Russia as a result of "increased pressure that we feel in Eastern Europe now and because of the assurance measures that we are taking in the Baltics, in Poland, in Romania."

"I believe there is a requirement for rotational forces in the future until we see the current situation begin to normalize," he said.

Breedlove said the halt in the conflict in Ukraine has been "pretty much a cease-fire in name only."

"There continue to be sporadic engagements in and around the cease-fire zone," he said. "And the second thing that I would say that has changed is we have seen a general trend towards a hardening of this line of demarcation and much more softening of the actual Ukraine-Russia border."

Russia’s border with Ukraine in the east is open and completely porous. As a result, Russian military equipment is flowing back and forth the border

"Russia continues to resupply the Russian-backed separatists in eastern Ukraine," Breedlove said.