NOVOAZOVSK, Ukraine — Two columns of Russian tanks and military vehicles fired Grad missiles at a border post in southeastern Ukraine, then rolled into the country Thursday as Ukraine’s overmatched border guards fled, a top Ukrainian official said.

The comments by Col. Andriy Lysenko, a spokesman for Ukraine’s National Security Council, and other statements from NATO, the pro-Russian separatists in Ukraine and the United States left no doubt that the Russian military had invaded southeastern Ukraine.

A top NATO official said at least ‘,000 Russian troops have poured into Ukraine with sophisticated equipment and have been in direct “contact” with Ukrainian soldiers, resulting in casualties. He called that a conservative estimate and said another 20,000 Russian troops were right over the Russian border.

“Russian forces have entered Ukraine,” Ukraine’s president declared Thursday, cancelling a foreign trip and calling an emergency meeting of his security council.

President Petro Poroshenko summoned the council as the strategic southeastern town of Novoazovsk appeared firmly under the control of separatists and their Russian backers, a new front in the war in eastern Ukraine between the separatists and Poroshenko’s government in Kiev.

“Today the president’s place is in Kiev,” Poroshenko said.

Lysenko said the missiles from Russia were fired about ‘’ a.m. and about an hour and a half later, two columns, including tanks and other fighting vehicles began an attack. They entered Ukraine from Veselo-Voznesenka and Maximovo of the Rostov region in Russia.

Russian stock markets dived as fears grew that the country was escalating its role in the conflict, a move that could provoke the U.S. and European Union to impose further sanctions on Russian businesses and individuals. Russia’s MICEX index dropped nearly 2 percent on Thursday, and major Russian state banks VTB and Sberbank dropped more than 4 percent.

Brig. Gen. Nico Tak told reporters at NATO headquarters that the ultimate aim of Russia was to stave off defeat for the separatists and turn eastern Ukraine into a “frozen conflict” that would destabilize the country “indefinitely.”

“Over the past two weeks we have noted a significant escalation in both the level and sophistication of Russia’s military interference in Ukraine,” Tak said in Casteau, Brussels. “Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favoring the Ukrainian military.”

NATO also produced satellite images to provide what it called “additional evidence that Russian combat soldiers, equipped with sophisticated heavy weaponry, are operating inside Ukraine’s sovereign territory.”