Rebel forces have also opened up a new front in the conflict in southern Ukraine. Heavy fighting was reported in the southeast of Ukraine near the town of Novoazovsk.
The UN Security Council met today and expressed outrage, although it said it couldn't verify the reports.
Russian U.N. Ambassador Vitaly Churkin offered a spirited defense, saying Kiev "is waging war against its own people."
U.S. State Department spokeswoman Jen Psaki said today, "It’s clear that Russia has not only stepped up its presence in eastern Ukraine and intervened directly with combat forces, armored vehicles, artillery and surface-to-air systems." But she declined to call Russia's actions an invasion and said there were no plans for additional sanctions on Russia.
“There are several different channels, all of which the United States is engaged with, that are having discussions about what the options are,” Psaki said.
President Obama also dodged an effort at a news conference to have him characterize Russia's actions as an invasion. "It's a continuation of what’s been taking place for months now," the president said.
Petro Poroshenko, who had met with Russian President Vladimir Putin earlier this week, canceled a visit to Turkey for the inauguration of newly elected president Recep Tayyip Erdogan and called a snap session of Ukraine's security council.
"I have decided to cancel my visit to Turkey because of the sharp escalation of the situation in the Donetsk region ... as Russian forces have entered Ukraine," he said.
Control of the area would give Russia a direct land corridor between Russia and the Crimean peninsula. The maneuver points to long-term strategic planning by Russia, a spokesman for the Ukrainian anti-terrorist command told ABC News.
Colonel Konstantin Hivrenko, spokesman for the Ukrainian Ministry of Defense, said Ukraine has “proof beyond any doubt” that Russian troops are now directly involved in the fighting.
“There are now steady and large-scale deliveries of Russian hardware. Armored convoys are crossing the border day and night,” he said. The Ukrainian Ministry of Defense also claims there are “uncountable” Russian tanks and artillery pieces at the Ukrainian border.
Ukrainian troops have been battling against separatist rebel forces in the region since April.
The prime minister of the self-proclaimed Donetsk People’s Republic, Alexander Zakharchenko, told Russian TV that 3,000 to 4,000 Russian soldiers were supporting the rebels.
“We have never hidden from anyone that there are many Russians amongst us,” he said. “Without their help, we would have struggled and it would have been more difficult to fight.”
In reaction to Russia’s continuing military build-up on NATO’s eastern flank, NATO Secretary General Anders Fogh Rasmussen announced the preparation of a Readiness Action Plan. In an interview with German Suedduetsche Zeitung, Rasmussen said NATO would form a quick reaction force to counter a possible Russian threat to countries on NATO’s eastern border.
ABC News' Ali Weinberg and the Associated Press contributed to this report
“Putin started a war in Europe,” he said.