Less than two days after Russian leader Vladimir Putin’s first meeting with newly elected Ukrainian President Petro Poroshenko, local and Western officials said Thursday thousands of Russian troops have streamed into Ukraine, forcing a tense situation to become, as NATO describes it, “increasingly grave.”

More than 10,000 Russian forces have been stationed on the Ukraine border for months, as pro-Russian groups believed to be supported by Moscow continue to lay siege to eastern Ukraine. The Pentagon has repeatedly expressed concern at the columns of tanks, infantry troops, artillery and air defense equipment it says could not be there only for training purposes, as the Russian government has said.

Satellite image of Russian forces in Ukraine. (Click to enlarge) NATO/DigitalGlobe

Reports of Russian forces operating openly or in disguise have circulated since civil unrest first began in Ukraine early this year. Thursday’s news marks the first time Western powers have openly stated Russian forces have invaded Ukraine.

NATO released satellite imagery “captured in late August” on Thursday that it says shows Russian combat forces and self-propelled artillery have crossed the border.

“Russian combat soldiers, equipped with sophisticated heavy weaponry, are operating inside Ukraine’s sovereign territory,” said Dutch Brig. Gen. Nico Tak, the director of NATO’s Comprehensive Crisis and Operations Management Center, according to a statement. "We have also detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred to separatist forces in eastern Ukraine.”

"The presence of these weapons along with substantial numbers of Russian combat troops inside Ukraine make the situation increasingly grave,” he said.

Russia’s actions are designed to reinforce and support the separatist fighters against the Ukrainian military, Tak said, which currently holds the momentum in the ongoing battle.

"Russia's ultimate aim is to alleviate pressure on separatist fighters in order to prolong this conflict indefinitely, which would result in further tragedy for the people of eastern Ukraine," he said.

Satellite image of Russian artillery in Ukraine. (Click to enlarge)

NATO/DigitalGlobe

The latest troop movement follows reports last week that a Russian convoy entered Ukraine under the guise of a humanitarian mission.

Pentagon spokesman Navy Rear Adm. John Kirby said Friday the Russian forces at the border are “very ready. They're very capable. They're very mobile. And they continue to do nothing but just increase the tension on the other side with Ukraine.”

He criticized any unilateral action by the Russian government -- humanitarian or otherwise -- without Kiev's express permission, calling it "a violation of Ukraine's sovereignty and territorial integrity by Russia."

Ukrainian President Poroshenko announced the Russian invasion early Thursday and said he would cancel his upcoming trip to Turkey, where he had planned to host a series of bilateral meetings to drum up international support.

He cited the “sharp worsening” of the situation in the eastern Donetsk region due to “the actual deployment of Russian troops on Ukrainian territory.”

Poroshenko called on the U.N. Security Council to convene a meeting to discuss what he called “the sharp deterioration of the situation in Ukraine.” He also asked for a similar session of the Council of the European Union.

The Security Council was scheduled to meet early Thursday afternoon in response to a formal request by Lithuania.

Kostiantyn Yeliseye, Ukraine’s representative to the EU, asked during an open session Thursday for large-scale military assistance “to stop the aggressor.”

Russia’s top diplomat to the European Union denied Russia had staged the invasion, and said Poroshenko’s claims stemmed from his dissatisfaction with his Tuesday meeting with Putin in Minsk.

“I think that this is related to the launch of the Minsk process, which, I believe, someone did not like. Indeed, there was no breakthrough at the summit,” said Vladimir Chizhov, Russia’s envoy to the EU, according to Russian state news service RIA Novosti. “Probably, someone in Kiev and in other places, not necessarily in Europe, did not like this.”

News of the Russian troop movement is simply “another deliberate information campaign that is not supported by evidence,” and a failure of Ukraine’s “so-called antiterrorist operation.”

State Department spokeswoman Jen Psaki offered Wednesday the U.S. concern over then reports of the Russian troop movement, but also cited a larger issue.