Russian military forces continued massing within six miles of Ukraine’s eastern border regions as tensions increased over Moscow’s so far bloodless attempt to take control of the country by force under the pretext of protecting ethnic Russians.

In Kiev, Ukrainian Foreign Ministry spokesman Yevhen Perebiynis said the massing of Russian forces near Ukraine’s eastern border appeared to be preparation for a military invasion, Ukraine’s Unian news agency reported.

Border troops reported Russian forces massing near borders in the Kharkiv, Luhansk, and Donetsk regions.

"Those actions taken by representatives of the Russian state authorities may be evidence that Russia is preparing for a possible invasion on Ukrainian territory through its land border with Ukraine," Perebiynis said.

Few details were available on troop movements but reports from the region indicated that nine convoys of armored vehicles were observed near the Russian border of Kharkiv, and five armored troop carriers were spotted less than two miles from the border in Donetsk. Military forces also were reported near the Luhansk administrative region.

So far, no land incursions have been reported.

"We are conducting permanent monitoring. It is true that there are [armed forces] deployed in the regions [bordering Ukraine]. They started exercises a week ago, and they are moving through their own territory. They haven't come close to the national border," Miron Sydor, head of the Border Guard's Eastern Regional Department, told Interfax.

Ukrainian Defense Minister Ihor Tenyukh said about 16,000 Russian troops currently are on Ukrainian territory. They include 5,500 special operations troops that were recently added to the troops based and recently transported to a Russian base in Crimea.

A Pentagon spokesman had no comment on the Russian troop movements.

In Washington, President Barack Obama and senior officials continued a rhetorical offensive calling on Russia to withdraw its troops from the region.

Obama said Russian troops in the Crimea are deeply troubling but he also offered conciliatory comments to Russia, telling reporters at the White House that Russian interests in Ukraine "can be recognized."

"What cannot be done is for Russia with impunity to put its soldiers on the ground and violate basic principles that are recognized around the world," Obama said during a meeting with Israeli Prime Minister Benjamin Netanyahu.

Obama said international outcry over the Russian military foray "indicates the degree to which Russia is on the wrong side of history."

The president said a series of diplomatic and economic steps are being prepared to "isolate" Russia.

Moscow officials and state-controlled news media rejected the threat of sanctions and warned that canceling the upcoming meeting of the G8 group of nations in Sochi next June would be a mistake.

A Russian Foreign Ministry statement said halting work on the G8 summit "harms not only the G8 countries, but also the whole international community, as Russian priorities include highly topical issues for the whole world."

Targeting the G8 meeting and freezing assets of Russian officials overseas would have little impact, state media reported.

The newspaper Komsomolskaya Pravda, a pro-Kremlin outlet, reported Monday that Moscow prepared in advance for western responses using economic and diplomatic means.

In other Russia-Ukraine developments:

Cyber attacks were reported underway Monday against Ukrainian media websites. They included distributed denial of service attacks that shut down websites of several news and broadcast outlets.

Vice President Joe Biden called Russian Prime Minister Dmitry Medvedev and urged Russia to pull its troops out. Defense Secretary Chuck Hagel called Russian Defense Minister Sergei Shogyu on Saturday and "expressed deep concern" about the Crimea intervention.

Russian Foreign Minister Sergei Lavrov said in Moscow that Russian troops would not be pulled back and were defending against what he called "ultra-nationalist threats" to ethnic Russians.

Ukraine’s Interior Ministry warned that Russian agents could conduct acts of provocation Monday night in the Crimea. The acts could involve people attempting to kill Russian military personnel in order to destabilize the region, Deputy Interior Minister Mykola Velichkovich told Kyiv 5 Kanal television.

Four Russian warships arrived in the Crimean port of Sevastopol on Saturday and an intelligence-gathering ship was photographed in the port on Sunday.

A Ukrainian Su-27 interceptor jet shadowed a Russian fight jet over the southern section of the Black Sea on two occasions Monday night, the Ukrainian Defense Ministry said.

A Chinese Foreign Ministry spokesman said China "understands" the situation in Ukraine and hopes for a "political solution" to the crisis. China in the past has opposed states that intervene in the internal affairs of other states.

Russia could lose access to a key nuclear missile facility in Dnepropetrovsk, Ukraine, where SS-18 intercontinental missiles are maintained, if a conflict breaks out, a U.S. official said.

Intelligence analysts who have compared Russian and Ukrainian military forces estimate that Moscow’s military is stronger than Ukraine’s forces. However, a regional conflict between the two states would be larger and last longer than the 2008 war between Russia and Georgia.

Russia’s 150,000 troops currently exercising in areas near Ukraine could be used for an attack and Moscow also could use its 7th Guards Airborne Division at Novorossiysk, along with special forces troops. Russia also has elite Interior Ministry troops in operations against Ukraine.

Ukraine’s military is largely built on Soviet-era forces inherited after the collapse of the Soviet Union in 1991 and Kiev has had limited military modernization in recent decades. Its forces are largely structured along the lines of Russian military forces.

Ukraine has limited forces in the Crimea, the center of Russian takeover efforts.

Ukraine is a NATO partner but has no security guarantees from the alliance.

On Sunday, Ukrainian Prime Minister Arseniy Yatsenyuk said the country was preparing for war.

"This is not a threat: this is actually the declaration of war to my country," Yatsenyuk said.

Influential Russian journalist Pavel Felgengauer, who U.S. officials say has good sources within the Russian military, reported on Monday that Russia leaders decided on military operations against Ukraine during security council meetings Feb. 21 and Feb. 25. At the second meeting a decision was made to send troops and forces into Crimea under the cover of military exercises designed to test combat readiness.

On Feb. 26, snap military exercises were launched involving forces from the Western and Central Military Districts, along with aerospace defense troops, airborne troops, long range aviation, and military transport aviation.

About 150,000 troops are taking part along with 90 aircraft, 120 helicopters, up to 880 tanks, 1,200 pieces of equipment, and 80 ships. Felgengauer said by exercising Central and Western military district forces the Russians sought to distract U.S. monitoring from the real objective: seizing the Crimea.

Shortly after the exercises were launched, Russian Il-76 troop transports began arriving at a Russian air base at Gvardeiskoye air base, north of the regional Crimean capital of Simferopol.

By Saturday, several thousand troops had been deployed.

Felgengauer said Russian forces are prepared to respond to a "serious regional war" possibly involving Western forces, despite official statements by Russian Defense Minister Sergei Shoygu that the exercises were not related to the Ukraine crisis.

The operation included the capture and blockade of strategic targets such as highways, airports, communications systems and media, along with Ukrainian military and government facilities.

Black Sea Fleet naval infantry and spetsnaz commanders were also involved.

Once the airports are under Russian military control, the plan calls for larger deployments of troops, he stated.

So far there has been no shooting and if it starts, that would be the pretext used by Moscow for an invasion.

Ukrainian military forces are being mobilized, the Kiev government announced. But so far military, police, and other security agencies have remained in their bases following the pro-democracy revolution that ousted pro-Russian President Viktor Yanukovich.