The UN Security Council met Thursday in an emergency session on the growing crisis in Ukraine with some members expressing outrage.

UN Undersecretary-General of Political Affairs Jeffrey Feltman told council members, as the meeting began, that the latest developments mark a "dangerous escalation in the conflict," but that the international body had no way of verifying the latest "deeply alarming reports."

The emergency Security Council meeting came hours after a top Ukrainian official said two columns of Russian tanks and military vehicles fired missiles from Russia at a Ukraine border post, then rolled into the country. That opened a new front in the war in eastern Ukraine between pro-Russia separatists and the new government of President Petro Poroshenko.

Statements from NATO, Poroshenko ,the separatists, the United States and the president of the Security Council left no doubt that Russia had invaded Ukraine. A top NATO official said at least 1,000 Russian troops have entered Ukraine with sophisticated equipment and have been in direct "contact" with Ukrainian soldiers, resulting in casualties.

The new southeastern front raised fears that the separatists are seeking to create a land link between Russia and Crimea, which Russia annexed in March.

U.S. Ambassador Samantha Power told the council. "Every single one has sent a straightforward, unified message: 'Russia, stop this conflict. Russia is not listening."'

"Russia has come before the council to say everything but the truth," Power said.

"We will continue working with G-7 partners to rachet up consequences on Russia," Power said, and France also threatened that sanctions will be increased if the escalation continues.

Prior to the meeting, Russian Ambassador to the UN Vitaly Churkin told reporters "You're at a loss," offering no further comment.

UK Ambassador Mark Lyall Grant told reporters: "Russia will be asked to explain why Russia has its troops inside Ukraine. It's very clear that Russian regular troops are now in Ukraine."

Lithuania's UN ambassador, Raimonda Murmokaite, who requested the emergency meeting, tweeted prior to its start: "An invasion is an invasion is an invasion."